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#!/usr/bin/perl
use warnings;
use strict;
my $file = "data.txt";
my $truncate_to = 100;
open READ, "$file" or die "Cannot open: $! \n";
while (<READ>) {
last if $. == $truncate_to;
}
my $size = tell READ;
print "$. lines read ($size bytes) \n";
exit if $. < $truncate_to; # already shorter
close READ;
print "Truncating to $size bytes...";
open WRITE, "+< $file" or die "Cannot write: $! \n";
truncate WRITE, $size;
print "done \n";
close WRITE;
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