Read the entire file into an array rather than opening and reading it inside the loop. Use
grep in scalar context to do the counting.
use strict;
use warnings;
use Data::Dumper;
my $file = \"a\nb\nb\nc\nc\nd\nd\nd\nf\nf\nf"; # In memory 'file'
my @ids = qw( a b c d e f g );
open my $FILE, '<', $file or die "cannot open input file";
my @lines = <$FILE>; # Read entire file into an array
close $FILE;
my %counts;
ID:
foreach my $id (@ids) {
my $count = grep { /$id/ } @lines;
if ($count) {
$counts{$id} = $count;
next ID;
}
warn "$id not found\n";
}
print Dumper \%counts;
OUTPUT:
e not found
g not found
$VAR1 = {
'd' => 3,
'a' => 1,
'c' => 2,
'f' => 3,
'b' => 2
};