Others have told you to declare the shared variable at the top of the file. Here's why:

#!/usr/bin/perl
use warnings;
use strict;

x();                 # this will trigger a warning

my $shared  = 'abc';

sub x {
    print "\$shared is: <$shared>\n";
}

x();                 # this will work fine
[download]

Although $shared is within the same scope as the sub x, its value has not yet been initialized. Therefore, the first time you call the subroutine, you get a warning for using an uninitialized value. Once the program flow passes over the shared point, everything it's OK. That was why, if you need a shared value, it should be "at the top".

HTH