Glenn,
Assuming you only want to do this on the first match:

#!/usr/bin/perl -w
use strict;

open (INPUT, "original") or die "Unable to open input file : $!";

open (OUTPUT, ">newfile") or die "Unable to open output file : $!";
select OUTPUT;

my ($match, $replaced);
while (<INPUT>) {
    $match = 1 if $_ =~ /baz/;
    if ($match && ! $replaced) {
        print "Replacement Line\n$_";
        ($match, $replaced) = (0, 1);
    }
    else {
        print;
    }
}
[download]

It is then a simple matter of using rename or File::Copy to move the new file in place. You may also want to take a look at Tie::File as it can treat a file as an array.

Cheers - L~R