A common way to do this for arbitrary literal string replacements would be:

my %subst = ( '&' => 'amp', '>' => 'gt', '<' => 'lt' );
my $alternatives = join('|', map quotemeta, sort { length($b) <=> leng
+th($a) || $b cmp $a } keys %subst);
$string =~ s/($alternatives)/$subst{$1}/ge;
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Have you considered HTML::Entities?

Hmm. I suppose if your input string were just ASCII, you could do tr///'s to carefully selected unicode characters, then turn off the utf8 flag and do a second tr to produce the desired letters. But that would be sick and twisted.