It does work. Well, it should. What are you really trying to do? It's hard to tell without seeing the rest of your code.

TT.pm (also taking into account ikegami's very good suggestion)

# -*- Perl -*-

use strict;
use warnings;

package TT;

use base 'Exporter';
our @EXPORT = qw/s1/;

sub s1 {
    my $sub = shift || \&s2;
    $sub->(qw/foo bar/);
}

sub s2 {
    print "Default s2\n";
    print "Args: @_\n";
}

1;


__END__
[download]

testTT.pl

#!/usr/bin/perl

use strict;
use warnings;
use TT;

# s1 with no args;
s1;

print "\n";

# s1 with explicit code passed in;
s1 sub {
    local $\="\n";
    print "Customized s2";
    print "Args:";
    print for @_;
};


__END__
[download]

(Try it yourself!) output:

Default s2
Args: foo bar

Customized s2
Args:
foo
bar
[download]

Incidentally, perldoc perlsub and perldoc perlref would help.