#!/usr/bin/perl
use warnings; # YOU FORGOT THESE!
use strict;

open HEADF, '<fastaheaders.txt' or
  die "cant open fastaheaders.txt: $!";
my @headers = <HEADF>;
close HEADF;

# create a regexp to search for all headers at once
# like this: (BLABLABLA|ASKDJFASD) will match either
# of the two.
my $regexp = '(' . join('|', @headers) . ')';

print 'Enter filename: ';
my $fasta_fn = <STDIN>;
chomp $fasta_fn; # remove newline, paranoia

open FASTAF, '<', $fasta_fn or die "can't open $fasta_fn: $!";
while(<FASTAF>) {
    # $_ = the line of text from the file now...

    # m!! is like using // to match text, m just tells
    # perl to use a different surrounding character: !

    # m!$regexp!o by itself matches against $_ automagically!
    # (the o says only read the value in $regexp once,
    #  it won't change anyway)
    # print by itself prints $_ automagically!
    print if(m!$regexp!o .. m!//! and not m!//!);
}
close FASTAF;