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while (<$IN>) {
     chomp;
...
     next unless /^.{3}\d{4}/; # lines of interest have 4 digits from 
+position 4 to 7
     # now the real processing ...
}

while (<$IN>) {
     chomp;
     next if /^#/ or /^\s*$/ or length < $min_useful_length or  /^REM/
+ or not /^.{3}\d{4}/ ...