#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;

my %hash = ( 'key1' => 'test1',
	     'key2' => 'test2',
	     'key3' => 'test3',
    );

print Dumper \%hash;

my @new_keys = keys %hash;
y/key[0-9]/alt[0-9]/ for @new_keys;
@hash{@new_keys} = delete @hash{keys %hash};

print Dumper \%hash;

__END__

$ perl test.pl
$VAR1 = {
          'key1' => 'test1',
          'key2' => 'test2',
          'key3' => 'test3'
        };
$VAR1 = {
          'alt3' => 'test3',
          'alt2' => 'test2',
          'alt1' => 'test1'
        };