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use warnings;
use strict;
...
    $max = $len if $len > $max;
}
print "$max\n";

a
abc
abcdef
abcd
abcde

              1         2         3        
     12345678901234567890123456789012345678

perl -nE'$l=y///c;$m=$l if$l>$m;END{say$m}' file.txt