I'd do it like this:

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

You might want to include a couple of "rename" steps after the outer while loop (rename the old "secondary.file" to "secondary.file.old", rename the output file to "secondary.file").

Note that if the given pattern never matches anything in the secondary file, the primary file never gets read or added to the output (output will be identical to input).