As several noted, BEGIN helps here. To further discourage such mistakes, I never write a scipt like you wrote. I write something more like:

use warnings;
use strict;

Main( @ARGV );
exit( 0 );

my %hash;
BEGIN {
    %hash = ( bar => 'value' );
}
sub Foo { print $hash{shift(@_)} }

sub Main
{
    Foo('bar');
}
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so that any code outside of subroutines but below the call to Main() never gets run (rather than being run when it might be too late).

Even more than my %hash = BEGIN { ... };, I'd like:

sub Foo
{
    my %hash = ( bar => 'value' ) BEGIN;
    print $hash{shift(@_)};
}
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That is, have BEGIN be available as a block-less statement modifier like 'if', 'for', and 'while'; which gives us "static" variables.