Better yet, here's a version that looks for prefixes lines as opposed to calls to print. 

use strict;
use warnings;

use Tie::Handle ();

package Tie::Handle::TimeStamp;

our @ISA = 'Tie::Handle';

sub wrap {
   my ($class, $globref) = @_;
   tie *$globref, $class, ">&=".fileno($globref);
}

sub TIEHANDLE {
   my $class = shift;
   my $fh    = \do { local *HANDLE };
   my $self  = bless({
      fh => $fh,
      nl => 1,
   }, $class);
   $self->OPEN(@_) if (@_);
   return $self;
}

sub EOF     { return eof($_[0]{fh})                }
sub TELL    { return tell($_[0]{fh})               }
sub FILENO  { return fileno($_[0]{fh})             }
sub SEEK    { return seek($_[0]{fh}, $_[1], $_[2]) } # hum...
sub CLOSE   { return close($_[0]{fh})              }
sub BINMODE { return binmode($_[0]{fh})            }

sub OPEN {
   my $self = $_[0];
   $self->CLOSE if defined($self->FILENO);
   return (@_ == 2
      ? open($self->{fh}, $_[1])
      : open($self->{fh}, $_[1], $_[2])
   );
}

sub WRITE {
   my $self = $_[0];
   my $len  = $_[2];
   my $text = substr($_[1], 0, $len);

   return 1 unless $len;

   my $fh = $self->{fh};
   my $nl = $self->{nl};
   my $lt;

   local ($,, $\);

   my $qsep = quotemeta($/);
   while ($text =~ /((?:(?!$qsep).)*(?:($qsep)|(?!$qsep).))/gs) {
      if ($nl) {
         $lt ||= "[" . localtime() . "] ";
         print $fh ($lt)
            or return 0;
      }

      print $fh $1
         or return 0;

      $nl = !!$2;
   }

   $self->{nl} = $nl;
   return 1;
}

1;

[download]

Note: Prints the time at which the caller started printing the line, not the time at which the caller finished printing the line.

Bug: Doesn't support zero length or undefined $/.

Bug: Doesn't properly detect the line ending if it's split over multiple prints.

Bug: Uses more memory than in should. 

local $/ = '||'
print('a|'); print('|b||');  # [timestamp] a||b||
print('a||b||');             # [timestamp] a||[timestamp] b||

[download]

Update: Added paren that was accidently deleted after testing.

Update: Simplified through the use of regexp. Tested to be safe.

Update: Changed $/ to $\.

Update: Added error checking, but I'm not sure that I'm returning the right value on error.

Update: Re-added support for $len which was accidently removed when I switched to regexps. Unfortunately, a copy is now made of the text to print.

Update: Switched from "\n" to $\ for splitting.

In reply to Re^2: Un "tie"ing a "tie" by ikegami
in thread Un "tie"ing a "tie" by herby1620

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