use strict;
use warnings;

open my $FILEC, '>', 'fileC';
open my $FILEB, '<', 'fileB';
open my $FILEA, '<', 'fileA';

while (my $line = <$FILEB>) {
    do {local $_; print $FILEC $_ while (<$FILEA>)} if $line =~ m/INSERT BEFORE HERE/;
    print $FILEC $line;
}