diotalevi's perfectly right.

Look at it this way:

my($ret) ||= foo();
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What you're doing amounts to this, step-wise:

1.  my ($ret);       # Create a scalar variable called $ret
2.  $ret ||= foo()   # If $ret is undefined or zero, assign it to foo(
+)
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But of COURSE $ret is undefined when you first declare it, because you haven't assigned it to anything!

A similarly bad thing is going on here:

my($ret) = $ret || foo();
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where you're trying to create a new scalar variable my($ret), but its value depends on a previously defined scalar called $ret (notice that's the same variable name?), or foo() if the the value of the $ret was previously undefined or zero.

If you use strict and warnings (as you always should), then you'll get an error with the second one.  Try this (essentially equivalent) code:

use strict;
use warnings;

my $x = $x || 5;
print "Now x = $x\n";
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Without strict and warnings you get:

Now x = 5
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but with them you get:

Global symbol "$x" requires explicit package name at test.pl line 5.
Execution of test.pl aborted due to compilation errors.
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Finally, the reason your title is misleading (as diotalevi suggests), is because it's perfectly fine to do either:

use strict;
use warnings;

my $y = 7;
my $x;

$x = $x || $y;
print "Now x = $x\n";
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and:

use strict;
use warnings;

my $y = 7;
my $x;

$x ||= $y;
print "Now x = $x\n";
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which will both yield the expected:

Now x = 7
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