#!/usr/bin/perl

use strict;
use warnings;

( @ARGV == 3 and -f $ARGV[0] )
    or die "Usage:  $0  primary.file  secondary.file  insertion_point_pattern\n";

open( my $in1, "<", $ARGV[0] ) or die "$ARGV[0]: $!\n";
open( my $in2, "<", $ARGV[1] ) or die "$ARGV[1]: $!\n";
open( my $out, ">", "$ARGV[1].including.$ARGV[0]" )
    or die "$ARGV[1].including.$ARGV[0]: $!\n";

while ( <$in2> ) {
    print $out $_;    # assuming you want content added after target pattern
    if ( /$ARGV[2]/ and $in1 ) {
        while ( <$in1> ) {
            print $out $_;
        }
        close $in1;
        $in1 = undef;
    }
}