You could wrap calls to the original and the added sub into a new anon sub:

$ perl -w
sub addcoderef { my ($orig, $add) = @_; sub { $orig->(); $add->(); } }
use strict;
my $coderef = sub { print "SUB1\n"; };
$coderef = addcoderef( $coderef, sub { print "SUB2\n"; } );
$coderef->();
__END__
SUB1
SUB2
[download]

but using an array scales better.

Another option is to deparse them (but deparsing historically hasn't worked 100%):

$ perl -w
use strict;
sub addcoderef {
   use B::Deparse;
   my $d = B::Deparse->new();
   eval join "\n", "sub {", map($d->coderef2text($_), @_), "}";
}
my $coderef = sub { print "SUB1\n"; };
$coderef = addcoderef( $coderef, sub { print "SUB2\n"; } );
$coderef->();
__END__
SUB1
SUB2
[download]