That works but better practice (i.e. coding style, etc.) would be:

#! /usr/bin/perl
use strict;
use warnings;

#////////////////////////////////////////
sub whoami {
    print "Just entered subroutine ", (caller(1))[3], "\n";
    return;
}

#////////////////////////////////////////
sub one_times_two {
    whoami();
    my( $one, $two ) = @_;
    return $one * $two;
}

#////////////////////////////////////////

my $one   = 12;
my $two   = 10;
my $three = one_times_two( $one, $two );
my $four  = 2 * $three;

print "2 x $three = $four\n";

exit 0;
[download]

With output:

Just entered subroutine main::one_times_two
2 x 120 = 240
[download]

Note: I beleive you need Perl 5.6 or greater for "use warnings".