http://www.perlmonks.org?node_id=561931


in reply to Rethrowing with die $@ considered harmful

Similarly, I've started noticing issues with the common practice of doing local $_. In some situations, that's not enough to prevent damage to the caller. Using local *_ fixes those problems. For example, consider the following function:

sub dequote { local $_ = @_ ? $_[0] : $_; s/^"//; s/"$//; s/\\(.)/$1/g; return $_; }

It assigns its parameter (or $_ if there are no parameters) to $_. It attempts to prevent damage to the caller by localizing $_ first, but I will show it doesn't always work.

Case 1

$_ is tied, or $_ is an alias to something which is tied.

Running
use strict; use warnings; sub dequote { local $_ = @_ ? $_[0] : $_; s/^"//; s/"$//; s/\\(.)/$1/g; return $_; } { package MyTie; sub TIESCALAR { bless(\my $var, shift) } sub FETCH { my $self = shift; print("FETCH\n"); return $$self; } sub STORE { my $self = shift; $$self = shift; print("STORE $$self\n"); return $$self;} } tie my $var, 'MyTie'; $var = '"John \"Foo\" Bar"'; print dequote, "\n" foreach $var;

gives

STORE "John \"Foo\" Bar" FETCH STORE FETCH Use of uninitialized value in concatenation (.) or string at 561931.pl + line 18. STORE FETCH Use of uninitialized value in substitution (s///) at 561931.pl line 6. FETCH Use of uninitialized value in substitution (s///) at 561931.pl line 7. FETCH Use of uninitialized value in substitution (s///) at 561931.pl line 8. FETCH STORE Use of uninitialized value in print at 561931.pl line 25.

Case 2

pos($_) is used by the caller.

Running
use strict; use warnings; sub dequote { local $_ = @_ ? $_[0] : $_; s/^"//; s/"$//; s/\\(.)/$1/g; return $_; } $_ = 'abcd'; /\G .. /gcx; print(pos(), "\n"); dequote('"John \"Foo\" Bar"'); print(pos(), "\n");

gives

2 Use of uninitialized value in print at 561931.pl line 18.

Changing
   local $_ = @_ ? $_[0] : $_;
to
   my $s = @_ ? $_[0] : $_;
   local *_ = \$s;
gives

2 2

The Solution

sub dequote { #local $_ = @_ ? $_[0] : $_; # XXX my $s = @_ ? $_[0] : $_; # Fix local *_ = \$s; # Fix s/^"//; s/"$//; s/\\(.)/$1/g; return $_; }

After making that change, the programs output
STORE "John \"Foo\" Bar" FETCH John "Foo" Bar

and

2 2

respectively.