As pointed out you really don't want to do this, but here is how anyway* ;-) This only works if you use the module not if you require if as when you use a module the modules import() method is called at the start of proceedings. This is not true for require when import is not called.

package MyModule;

use strict;
use Exporter;
use vars qw($VERSION @ISA @EXPORT $MOD_CALL );

$VERSION     = 1.00;
@ISA         = qw(Exporter);
@EXPORT      = qw( func1 );

sub func1  { return reverse @_  }

sub import { $MOD_CALL++; }

CHECK{ print $MOD_CALL ? "Called as a module\n" : "Called as a script\
+n" }

1;
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On the rare occasions I have made a module a stand alone executable (usually for all the wrong laziness reasons) I have just used the presence of values in @ARGV to initiate script like behaviour. Generally you want to pass some data to a script anyway......

if ( @ARGV ) {
    print "I'm a script!\n";
    # do stuff
  exit 0;
}
# only get to here if you we are a module
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* Recall the story about getting enough rope to hang yourself?