You aren't showing the whole script, so I wonder... do you have these lines anywhere in your code (near the top)? 
use strict;
use warnings;

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Apart from that, I wonder if your "readmap" command ever outputs any lines with initial whitespace. If so, your split statement should be: 
    @fields = spit " ";

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because that will differ from /\s+/ -- try this snippet to see the difference: 
$_ = "   begins with whitespace";

@a = split " ";
@b = split /\s+/;

print "quoted-space split returns ", scalar @a, " elements\n";
print "\\s+ split returns ", scalar @b, ", first one has length ",leng
+th($b[0]),"\n";

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update: To clarify the issue, using "\s+" for splitting will get you into real trouble if the "readmap" output varies in terms of presence/absence of whitespace at the beginning of each line. If the output is consistent in this regard, then using "\s+" is probably not a problem -- you just need to make sure you've counted the field indexes correctly, so that $fields[2] etc really point at what you want them to point at.

In reply to Re: Working out frequency statistics with perl by graff
in thread Working out frequency statistics with perl by wishartz

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