As I understand it, attributes are syntactic sugar to do "stuff". Here's a random simple example:

package Loud;
use Attribute::Handlers;

sub Loud :ATTR {
  my ($package, $symbol, $referent, $attr, $data, $phase) = @_;
  no strict 'refs';

  # redefines subroutines given the "Loud" attribute
  *{$package.'::'.*{$symbol}{NAME}} = sub {

    # dup STDOUT and instead pipe things to another program
    open my $oldout, ">&STDOUT"             or die "Can't dup STDOUT: 
+$!";
    open STDOUT, "| perl -pe 'tr/a-z/A-Z/'" or die "Can't redirect STD
+OUT: $!";
    $| = 1;

    # call the original subroutine
    $referent->(@_);

    # restore STDOUT
    close STDOUT;
    open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
  }
}


# ... in another file or package
package main;
use base qw/Loud/;

# We want this function to be loud
sub foo : Loud {
  print "Purple is a nice color.\n";
}

foo();
print "Don't yell!\n";
[download]

The output of this program (it runs as shown above) is:

PURPLE IS A NICE COLOR.
Don't yell!
[download]

Catalyst uses the ActionClass attribute here to append some action (provided by RenderView.pm) to your subroutine