It's easy to do if you know the classname at compile time. It's possible, but slightly more difficult if you don't know until runtime.

#!/usr/bin/perl -w 

use strict;

package Foo;

sub new {
    bless({}, $_[0]);
}

sub do_it {
    print "Doing something for $_[0]!\n";
}

package main;

my $foo = new Foo;
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The direct approach just needs a good reference.

my $do_it = \&Foo::do_it;
$do_it->('direct call');
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The other way requires a bit of magic to get the proper class name and build something roughly equivalent to the last approach.

I like using eval for this sort of thing. Your mileage may vary, and you can go far with a typeglob.

my $ref = ref($foo);
my $do_it2;
eval "\$do_it2 = \\&${ref}::do_it";
$do_it2->('indirect call');
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The eval uses backslashes to make sure $do_it2 isn't interpolated and the ampersand isn't escaped. The braces around foo let the interpreter know that we only want $ref not $ref::do_it ($do_it in the package named 'ref').

Update: If I took the time to stop and to think about it, I would use can as the brilliant and thoughtful btrott so sagely postulates.