Since you have asked a question which is basically a logical problem rather than perl specific problem

Find below a simple piece of code

$stringtomatch = "overload";
$strlen = 8;
$strlen = -8;#negative length to truncate line 
while(<$file>){
$line = $_; #($_ is a special perl var , look it up)
$lineslice = substr $line, 0, $strlen;
#now go and do a match for the rest of the line 
}
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