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#!/usr/bin/perl -wi.bak
use strict;

my %params;
my $filename = shift || die "Usage: netswap.pl CONFILENAME\n";

open(FH,"$filename")
    or die "Could not open network configuration file: [$filename] for
+ reading: $!\n";

while(<FH>){
    # Skip the blank lines or the commented lines
...
    # avoid typing mistakes
    $params{$name} = lc($value);
}

print <<"EOL";
new hostname    = [$params{hostname}]
...
new gateway     = [$params{gateway}]

EOL

my $replacements = {
    '/etc/hosts' => {
...
    my $replacements = shift;
    foreach my $filename (keys %{$replacements}){
        print STDERR "Processing file [$filename]\n";

        unless (-e $filename){
            print STDERR "  [$filename] cannot be found!\n";
            next;
        }

        @ARGV=($filename); # put the filename where perl can find it.
        while(<>){ 
...
        print STDERR "  done\n";
    }
}