Here's a test, which shows the same behaviour happening as for arrays:

#!/usr/bin/perl

use Data::Dumper;

my %h = ( aa => hello, bb => ghello );

foreach ( values %h ) { s/el/ig/g };

print Dumper( \%h );
[download]

Output:

$VAR1 = {
          'aa' => 'higlo',
          'bb' => 'ghiglo'
        };
[download]