#!/usr/bin/perl

use strict;

my @repl_map;
$repl_map[1] = { a => 'k', b => 'l', c => 'm' };
$repl_map[4] = { x => 'i', y => 'j', z => 'k' };
# positions 0,2,3 are undef (to be left unchanged)
# positions >=5 (if present) will also be unchanged

my $match = "^(";  # note "^" to anchor match at offset 0
my $replc = "";
my $captr = 1;
for my $pos ( 0 .. $#repl_map ) {
    if ( not defined( $repl_map[$pos] )) {
        $match .= ".";
    }
    else {
        warn sprintf( " mapping at pos %d: %s\n", $pos,
                      join( ", ", map { "$_ => $repl_map[$pos]{$_}" }
                                 keys %{$repl_map[$pos]} ));

        $match .= ")([" .
                  join("", sort keys %{$repl_map[$pos]}) .
                  "])(";
        $replc .= sprintf( "\$%d\$repl_map[%d]{\$%d}",
                           $captr, $pos, $captr+1 );
        $captr += 2;
    }
}
chop $match;  # remove trailing "("
warn " match: $match;  replc: $replc\n\n";

my @strings = qw/aaaxxx bbbyyy ccczzz/;
print "BEFORE: @strings\n";
for ( @strings ) {
    eval "s/$match/$replc/";
}
print "AFTER:  @strings\n";


##</code><code>##

 mapping at pos 1: c => m, a => k, b => l
 mapping at pos 4: y => j, x => i, z => k
 match: ^(.)([abc])(..)([xyz]);  replc: $1$repl_map[1]{$2}$3$repl_map[4]{$4}


##</code><code>##

BEFORE: aaaxxx bbbyyy ccczzz
AFTER:  akaxix blbyjy cmczkz