I suspect you can get most of what you want with require and perlboot. If that isn't helpful, please tell me how a plugin differs from a perl package and I will refine my answer.

UPDATE: some (untested) code as requested:

for my $potential (<plugin_dir/*.pm>) {
    eval qq( require "$potential" );
    if( $@ ) {
        warn "plugin failed to load: $potential";
    }
    else {
        if( $potential =~ m|plugin_dir/(.+?)\.pm| ) {
          my $c = $1;
          if( eval { $c->can("expected_method1") and $c->can("somethin
+g_else") } ) {
            push @loaded_plugins, $c;

          } else {
            warn "$potential failed to load correctly";
        }
    }
}
[download]

Although, rather than rolling your own — which I barely started above, you'd be better off looking into some of the posts below. Of particular interest is "Re: How to make plugins."