Any suggestions as to how to simplify/condense this code snippet ?

As opposed to condensing it, if I came across this code I'd rather it be expanded:

    my %Y;
 
    while (<>) {
        ...
        my ($y) = ...;
        next unless $y;

        if (@ARGV) {
            for (@ARGV) { 
                $Y{$y}++;
                next if $y eq $_;  
            }
        } 
        else { 
            $Y{$y}++; 
        }
    }
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As far as changes, I agree with the implementation of more descriptive variable names. I'm also picky about unless for some weird, unconfirmed reason (probably because 'unless' isn't seen in other languages and/or the '!' stands out loudly), so I'd write that as next if ! $y, but I digress.

One simplification that can condense things a bit is changing this:

    if (@ARGV) {
        for (@ARGV) { 
            $Y{$y}++;
            next if $y eq $_;  
        }
    }
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... to this:

    for (@ARGV) { 
        $Y{$y}++;
        next if $y eq $_;  
    }
[download]

In other words, unless you explicitly need to, there's no need to check whether the @ARGV variable is true before you attempt to iterate over it. The for()/foreach() implicitly does this for you.