open my $IFILE, '<', $IDFILE || die "Could not open $IDFILE: $!";

##</code><code>##

while ( my $ifile_line = <$IFILE> ) {
    my $hits = substr($ifile_line, 0, 6);
    push @id_hits, $hits;
}

##</code><code>##

my @id_hits=map substr($_, 0, 6), <$IFILE>;