my %map = ( a=>1, ab=>23, cd=>45 );   # 1.
my ($regex) = map { qr/$_/ }          # 2.
    join '|', map {quotemeta}
    sort { length $b <=> length $a
           or $a cmp $b }             # 3.
    keys %map;
print "$regex\n";                     # 4.
# Now, use the regex
my @strings = qw/ abcd aacd abaab /;  # 5.
for (@strings) {
    my $before = $_;
    s/($regex)/$map{$1}/g;            # 6.
    print "$before -> $_\n";          # 7.
}