I've run into this strange behaviour when playing with trying to make functions return different values from the number of parameters they were called with. If you test @_, and that you return from a do { ... } block where a variable is localized, the return value is undef if you don't add an else statement.

#!/usr/bin/perl -w

use strict;

sub foo {
  if (@_) {
    return do { my $dummy; 1; };
   } else {
    return 0;
  }
}

# let's just remove the else

sub bar {
  if (@_) {
     return do { my $dummy; 1; };
  }
  return 0;
}

print foo().' '.foo('baz')."\n"; # that was expected
print bar().' '.bar('baz')."\n"; # undef
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##################################################
Expected output:

0 1
0 1

##################################################
Actual output:

0 1
Use of uninitialized value in concatenation (.) or string at ./undef.p
+l line 24.
0

##################################################
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2006-10-07 Unapproved by planetscape once evidence of habitual plagiarism uncovered.