use strict;
use warnings;
use 5.010;

sub routine {
    my %hash;
    BEGIN {
        $hash{KEY} = "string";
    }
    $hash{FOO} = "something else";
    say join " ", keys %hash;
}

routine; routine; routine;

__END__
Variable "%hash" will not stay shared at /tmp/hh line 12.
KEY FOO
FOO
FOO