#!/usr/bin/perl -l
use strict;
use warnings;
my %replace = qw/pre 1 prefix 2/;
my $norev = '('.join('|', sort keys %replace).')';
my $withrev = '('.join('|', reverse sort keys %replace).')';
while (<DATA>) {
    chomp;
    print "Line: $_";
    print "norev <$1>" if /$norev/;
    print "w/rev <$1>" if /$withrev/;
}
__END__
This line contains prefix (which also matches 'pre')