my $regex = "s/$old/$new/g";

##</code><code>##

#!/usr/bin/perl

use strict;
use Getopt::Long;

# ... define arrays of file names, then ...

my ( $opt_ip, $opt_host );
GetOptions( 'ip=s' => \$opt_ip, 'host=s' => \$opt_host );

my $Usage = "Usage:  $0 [-ip old:new] [-host old:new]\n";
die $Usage unless ( $opt_ip =~ /\S:\S/ or $opt_host =~ /\S:\S/ );

my $ipreg = qr/\d{1,3}(?:\.\d{1,3}){3}/;
my $hostreg = qr/\w+(?:\.\w+){2,}/;

if ( $opt_ip ) {
    $opt_ip =~ /($ipreg):($ipreg)/ or die "Bad value for -ip: $opt_ip\n $Usage";
    my ( $old, $new ) = ( $1, $2 );
    doReplace( $old, $new, 'IP', \@ipFiles );

    s/.\d+$// for ( $old, $new );
    doReplace( $old, $new, 'RTR', \@routerfiles ) if ( $old ne $new );
}
if ( $opt_host ) {
    $opt_host =~ /($hostreg):($hostreg)/ or die "Bad value for -host: $opt_host\n $Usage";
    my ( $old, $new ) = ( $1, $2 );
    doReplace( $old, $new, 'HOST', \@hostFiles );
}

sub doReplace
{
    my ( $old, $new, $typ, $files ) = @_;
    open TMP,">/tmp/$typ-config-editor.$$.perl" or die "can't write script file: $!"
    print TMP "s{\\b\\Q$old\\E\\b}{$new}\n";
    close TMP;

    for my $f ( @$files ) {
        my $cmd = "perl -i .bak.$$ -p /tmp/$typ-config-editor.$$.perl $f";
        print "cmd: $cmd\n";
        # system( $cmd ) or print "\t $f changed\n";
        # (system returns exit status of $cmd: 0 for success)
    }
}