You can increment the count as you read lines from your input file:

use warnings;
use strict;

my $cnt = 0;
while (<DATA>) {
    chomp;
    my ($an, $dist) = split;
    $cnt++ if $dist < 10;
}
print "cnt=$cnt\n";

__DATA__
a 50
b 15
c 9
d 11
e 33
f 2
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Prints:

cnt=2
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