use strict;
use warnings;
use autodie;

my ( %ids, $id );

open my $idFH, '<', 'sample_IDs.txt';
while (<$idFH>) {
	$ids{$1} = $2 if /(.+)\s+(.+)/;
}
close $idFH;

open my $sampleFH, '<', 'sample_reads.fasta';
while (<$sampleFH>) {
	s/\n/"_weight=$ids{$id}\n"/e
	  if ($id) = /^(>\S+)/ and exists $ids{$id};
	print;
}
close $sampleFH;