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

my $file = "myfile";

open(FILE,$file);
foreach (<FILE>) {

    $_ =~ s/\<REPLACE_TEXT_1\>/&replace_text_1/ if $_ =~ m/\<REPLACE_TEXT_1\>/;
    print "$_";

}
close(FILE);

sub replace_text_1 {

    #some stuff
    print "foobar";
    #some other stuff

}