use strict;

sub vert {
   my ($arg) = @_;
   print "Arg is $arg\n";
   return 'text';
}

# Correct way of creating subroutine reference
my $sub_ref = \|

# Will print "Arg is foo"
&$sub_ref('foo');

# Incorrect attempt at creating a subroutine reference
# Will print "Arg is bar" as a side effect
my $not_sub_ref = \&vert('bar');

# Will die horribly, since $not_sub_ref is a reference
# to the string 'text' (the return value of &vert),
# not a reference to a subroutine
&$not_sub_ref();