#! /usr/bin/perl  -pi.bak

# quick and dirty script
# please consult perldoc perlrun for information about the used switches -p -i
# or online at: http://perldoc.perl.org/perlrun.html

use strict;
use warnings;

use vars qw( $search $replace );

# set some values at the start of program
BEGIN {
    $search  = "abc\nabc\nabc";
    $replace = "123\n123\n123";

    # make us read a complete file at once
    $/ = undef;
}

# the following code is run every time a new line or dataset is read
# beware of the -p switch on shebang.
# each modified datafile is backed up before; see -i.bak on shebang

s/$search/$replace/msg;