#!perl -w
use strict;

my %hash = ( a => 1,
             b => 2,
             c => 3 );

sub change {
    my $hash = shift;
    $hash->{c} = 4;
}

change( \%hash );

for my $key (sort keys %hash) {
    print "$key : $hash{$key}\n";
}

__END__
Output looks like:
a : 1
b : 2
c : 4