As others have already said, %INC.

However, note that it's quite possible to directly manipulate this hash:

use 5.010;

# Trick perl into thinking that Moose.pm has
# already been required.
$INC{'Moose.pm'} = '/tmp/Moose.pm';

# Require it for real. But perl does nothing!
require Moose;

# Perl thinks Moose is loaded.
say "Moose is loaded" if $INC{'Moose.pm'};

# But it isn't really, so we don't know VERSION.
say 'Moose version:', Moose->VERSION;
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I have in the past had cause to directly manipulate %INC. A module I was using wanted to be passed the name of plugin module to use. It would require that plugin and then use it. However, I wanted to define the plugin inline in my code, a la

{
  package My::Plugin;
  ...
}

Something::That::Uses::Plugins->load_plugin('My::Plugin');
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But this caused Something::That::Uses::Plugins to attempt to require the non-existent file "My/Plugin.pm". So...

{
  package My::Plugin;
  ...
  $INC{'My/Plugin.pm'} = __FILE__;
}

Something::That::Uses::Plugins->load_plugin('My::Plugin');
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And everybody's happy.

But anyway, my point is that although %INC should give you the information you want, it can't necessarily be trusted 100%.