A possible solution is to get the old value of the binding and fire this callback in the new callback. E.g.

#!/usr/bin/perl
use Tk;
$mw = tkinit;
$mw->bind("<Return>" => sub { warn "Old binding" });
my $old_binding = $mw->bind("<Return>");
$mw->bind("<Return>" => sub { $old_binding->Call();
                              warn "New binding" });
MainLoop;
__END__
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Note that the return value of bind is not a CODE reference, but a Tk::Callback object, so you have to use the Call() method.

It's also possible to add a new bindtag via the bindtags() method:

#!/usr/bin/perl
use Tk;
$mw = tkinit;
$mw->bind("<Return>" => sub { warn "Old binding" });
$mw->bind("anotherbindtag", "<Return>" => sub { warn "New binding" });
$mw->bindtags(["anotherbindtag", $mw->bindtags]);
MainLoop;
__END__
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