#!/usr/bin/perl -w

open(FILE, "+</tmp/foo.txt") || die "Can't open: $!";
$l=0;
@lines=map { chomp;$l=$l<length()?length():$l;$_ } <FILE>;
seek(FILE, 0, 0) || warn "Seek: $!";

for(@lines) {
        printf FILE "%-${l}s\n", $_;
}