Given this little snippet, it appears that the goal of the first loop is to remove all the lines that don't end with 0 and put those into an array. The next loop then reads that array. So, you're iterating over your data twice, but you don't need to. You've got 1 completely copy of the file in memory, and another mostly completely copy. It takes time to create and destroy those copies. You don't need to do that either.

open(IN,$my_file) || die "Can't open: $!";
while (my $line = <IN>) {
    next unless $line =~ /0\s*$/;
    $line =~ /^(\d+.?\d*)//;
    if ($1 > .5) {
        # etc, etc
    }
}
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here's the break down line by line

• Open the file
• Process each line in turn. So, we're not storing the whole thing in memory (takes time to allocate said memory)
• The if $1 == 0 part of your first loop only kept lines that ended in 0 for @zeroat. Just test directly for the line ending in 0 and throw away the rest.
• no need for the regexp to catch the last character, we already know it's a 0
• Test only the > .5 part, again no need for the $2 == 0 since we already threw away every line that didn't end in 0.
• Rest of processing

HTH

/\/\averick
OmG! They killed tilly! You *bleep*!!