my $old = 'hat|coat|shoe';
my %new = ( cap    => 'hat',
            jacket => 'coat',
            shoe   => 'boot',
           );
for my $word ( @words ) {
    $word =~ tr/[A-Za-z]/[M-ZA-Lm-za-l]/;  # decode ROT13
    $word =~ s/($old)/$new{$1}/ei;         # convert cheap clothes into expensive ones
}