Unexpected results when removing a HERE-DOC "ending" newline, with map and/or s/// depending on context.

[olivierp]

Dear monks,
While searching for a way to remove the "last" newline in a here-doc, i.e. the one ahead of the end tag, I stumbled across behaviour(s) I do not understand, for which I seek enlightenment
I am probably missing something obvious...
Consider these snippets: (the complete script is at the end of the post)

(my $var1 = <<EOT) =~ s/\r?\n/(munched)/g;
First line of "var1"
Second line of "var1" has a newline here ->
EOT

(my $var2 = <<EOT) =~ s/\r?\n\Z/(munched)/;
First line of "var2"
Second line of "var2" had a newline here ->
EOT

Nothing unexpected here. The first replaces every newline, not what I want, and the second is part of what I need.

Now something strange:Fixed following fglock's and Errto's comments.

# Was building $var3 in scalar instead of list context
# my $var3 = map { s/\r?\n\Z/(munched)/; $_ } <<EOT;
my ($var3) = map { s/\r?\n\Z/(munched)/; $_ } <<EOT;
Var also has an interpolated var:
       $interpolated
And the last newline is also absent ->
EOT

var3 now contains the "count" of substitutions. I don't understand why,
If I change the above to a here-doc with no interpolated var(s), as here:

my $var4;
# This generates "Modification of a read-only value attempted at";
eval {
  # Still dies, even when in list context.
  # $var4 = map { s/\r?\n\Z/(munched)/; $_ } <<EOT
  ($var4) = map { s/\r?\n\Z/(munched)/; $_ } <<EOT
var4 has _NO_ interpolated var,
and will generate a runtime error
EOT
};

if ($@){
  ($var4 = $@) =~ s/\r?\n\Z//;
  $@ = undef;
  print map {s/\r?\n\Z//;$_ }(${^__} =<<EOT);
The error generated by
\$var4 = map { s/\\r?\\n\\Z/(munched)/; \$_ } <<EOT;
is: $var4

EOT
}

I get Modification of a read-only value attempted at... Why is $var4's here-doc considered read-only, when the only difference with $var3's is the $interpolated item ?

While I could do with my working snippet, I actually want to do this while creating a HoH. And this is where I stumble again on things I cannot grasp:

# map{}' on the last key is ok.
my $href = {
        key1 => {
             subkey1 => "Defined first is OK",
             subkey2 => map{s/\r?\n\Z/(munched)/;$_} (${^__}=<<EOT),
First line in sub key.
Second line. The third contains an interpolated value, and all is OK.
Line 3    $interpolated
and no newline here ->
EOT
           },
       };

# This explodes...
# map{}' ing, then defining another key leads to a runtime error.
eval {
$href->{key2} = {
         subkey1 => map{s/\r?\n\Z/(munched)/;$_} ($ {^__}=<<EOT),
First line in sub key.
Second line. The third contains an interpolated value, and all is OK.
Line 3    $var3
and no newline here ->
EOT
           subkey2 => "Kaboom !",
          };
};

if ($@) {
  ($href->{key2}->{subkey1} = $@) =~ s/\r?\n\Z//;
  $href->{key2}->{subkey2} = "Set after eval";
  $@ = undef;
}

Why is it that using map during the definition of the first key is OK, and if used in another key definition blows up ?
In both cases I do have an interpolated value, and I think that ${^__} is not a read-only value.

Now, let's try with something "cleaner" than map:

# This doesn't die, but gives the count of matches...
$href->{key3} = {
         subkey1 => "Hello",
         subkey2 => ($ {^__} =<<EOT) =~  s/\r?\n\Z/(munched)/,
This works fine,
and also strips
my ending newline ->
EOT
         subkey3 => "No explosion here...but the count of matches",
        };

Here again, I get a count of substitutions, and do not understand why.
And finally the las thing I tried, that does what I want:

$href->{key4} = {
         subkey1 => "Hello",
         subkey2 => eval {
           ($ {^__} =<<EOT) =~ s/\r?\n\Z/(munched)/;
This works fine,
and also strips
my ending newline ->
EOT
           return $ {^__};
         },
         subkey3 => "No explosion here...",
        };

Would someone kindly explain what I am messing up ?
Update:
To answer why I would map or s/// a here-doc, I am dealing with window's WMI Query language, which does not appreciate having a newline in certain places of its queries. In particular, the condition in a WHERE clause:
WHERE Path='something' is OK.
WHERe Path='
something' is not.
Complete script here:

use strict;
use warnings;

my $interpolated = "I was interpolated";

# This strips _every_ newline
(my $var1 = <<EOT) =~ s/\r?\n/(munched)/g;
First line of "var1"
Second line of "var1" has a newline here ->
EOT

#This only strips the "ending" newline
(my $var2 = <<EOT) =~ s/\r?\n\Z/(munched)/;
First line of "var2"
Second line of "var2" had a newline here ->
EOT

# This will return the number of substitutions
# Fixed. Was building $var3 in scalar instead of list context. 
#my $var3 = map { s/\r?\n\Z/(munched)/; $_ } <<EOT;
my ($var3) = map { s/\r?\n\Z/(munched)/; $_ } <<EOT;
Var also has an interpolated var:
       $interpolated
And the last newline is also absent ->
EOT

my $var4;
# This generates "Modification of a read-only value attempted at";
eval {
  # Still dies, even if called in list context.
  # $var4 = map { s/\r?\n\Z/(munched)/; $_ } <<EOT;
  ($var4) = map { s/\r?\n\Z/(munched)/; $_ } <<EOT;
var4 also has _NO_ interpolated var,
and will generate a runtime error
EOT
};

if ($@){
  ($var4 = $@) =~ s/\r?\n\Z//;
  $@ = undef;
  print map {s/\r?\n\Z//;$_ }(${^__} =<<EOT);
The error generated by
\$var4 = map { s/\\r?\\n\\Z/(munched)/; \$_ } <<EOT;
is: $var4

EOT
}


# map{}' on the last key is ok.
my $href = {
        key1 => {
             subkey1 => "Defined first is OK",
             subkey2 => map{s/\r?\n\Z/(munched)/;$_} (${^__}=<<EOT),
First line in sub key.
Second line. The third contains an interpolated value, and all is OK.
Line 3    $interpolated
and no newline here ->
EOT
           },
       };

# This explodes...
# map{}' ing, then defining another key leads to a runtime error.
eval {
$href->{key2} = {
         subkey1 => map{s/\r?\n\Z/(munched)/;$_} ($ {^__}=<<EOT),
First line in sub key.
Second line. The third contains an interpolated value, and all is OK.
Line 3    $var3
and no newline here ->
EOT
           subkey2 => "Kaboom !",
          };
};

if ($@) {
  ($href->{key2}->{subkey1} = $@) =~ s/\r?\n\Z//;
  $href->{key2}->{subkey2} = "Set after eval";
  $@ = undef;
}

# This doesn't die, but gives the count of matches...
$href->{key3} = {
         subkey1 => "Hello",
         subkey2 => ($ {^__} =<<EOT) =~  s/\r?\n\Z/(munched)/,
This works fine,
and also strips
my ending newline ->
EOT
         subkey3 => "No explosion here...but the count of matches abov
+e",
        };

$href->{key4} = {
         subkey1 => "Hello",
         subkey2 => eval {
           ($ {^__} =<<EOT) =~ s/\r?\n\Z/(munched)/;
This works fine,
and also strips
my ending newline ->
EOT
           return $ {^__};
         },
         subkey3 => "No explosion here...",
        };

print map {s/\r?\n\Z//; $_} <<EOF;
var1 :
<$var1>
------
var2 :
<$var2>
------
var3 :
<$var3>
------
var4 :
<$var4>
------
From the hash refs:
key1->subkey1
<$href->{key1}->{subkey1}>
key1->subkey2
<$href->{key1}->{subkey2}>
------
key2->subkey1
<$href->{key2}->{subkey1}>
key2->subkey2
<$href->{key2}->{subkey2}>
------
key3->subkey1
<$href->{key3}->{subkey1}>
key3->subkey2
<$href->{key3}->{subkey2}>
key3->subkey3
<$href->{key3}->{subkey3}>
-----
key4->subkey1
<$href->{key4}->{subkey1}>
key4->subkey2
<$href->{key4}->{subkey2}>
key4->subkey3
<$href->{key4}->{subkey3}>
EOF