Further to choroba's point about warnings and Athanasius's point 1 about prototypes (++both): Using warnings would have alerted you the fact that there was more than one unkosher thing about the way you were using prototypes. Without warnings:

c:\@Work\Perl\monks>perl -le
"use strict;
 ;;
 foo();
 sub foo (my $x) { my $x = 'hiya';  print $x; }
"
hiya
[download]

And with:

c:\@Work\Perl\monks>perl -le
"use strict;
 use warnings;
 ;;
 foo();
 sub foo (my $x) { my $x = 'hiya';  print $x; }
"
Illegal character in prototype for main::foo : my$x at -e line 1.
main::foo() called too early to check prototype at -e line 1.
hiya
[download]