#!/usr/bin/perl -w
use strict;

my (%hash1, %hash2);    # Defined elsewhere in the program

for my $key_h1 (keys %hash1) {
    my $val_h1 = $hash1{$key_h1};
    for my $key_h2 (keys %hash2) {
        my $val_h2 = $hash2{$key_h2}
        
        # You can now check $key_h1/2 & $val_h1/2
        # And safely modify %hash1 or %hash2
    }
}