Have you tried adding some print statements showing the values of your variables at each pass? This should help a lot with debugging (unless you are savvy with the debugger--but then why are you asking us! ;p) For instance, at loops and conditional statements, add something like:

print <<END_DEBUG if $DEBUG;

\$m   = $m
\$n   = $n
\$j   = $j
\$row = $row

END_DEBUG
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The variables $m and $n are easy to mix up... if you didn't cut and paste your exact code, are you sure you haven't accidentally typed $m++ rather than $n++ somewhere? The debug output will help you locate mistakes that you can't find while staring at the code.

(Don't forget to put somewhere near the top of your code)

my $DEBUG = 1;
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