You want something like this:

#!/usr/bin/perl
use warnings;
use strict;
use POSIX qw/ strftime /;


my $source = 'libsource.txt';
my $dest   = 'library2.txt';

open SOURCE, '<', $source or die "Cannot open '$source' $!";
open DEST,   '>', $dest   or die "Cannot open '$dest' $!";

while ( <SOURCE> ) {
    s{\b(\d{9,10})\b}{ strftime '%m/%d/%Y', localtime $1 }eg;
    print DEST;
    }

close SOURCE;
close DEST;
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