use strict;

open( IN, "file.txt" ) or die "file.txt: $!";
open( OUT, "out.txt" ) or die "out.txt: $!";

# or: open( my $in, "file.txt") ...

while (<IN>) {  # or: while (<$in> ...)
    $_ = join "", map { sprintf("%2.2x",ord($_)) } split //;
    print OUT $_, $/;
}

##</code><code>##

    my $outstr = "";
    $outstr .= sprintf("%2.2x",ord($_)) for (split //);
    print $outstr, $/;