use strict;
use warnings;

my $func = \&main::non_defined_code;

if( function_exists( $func )) { print 'YES'; }
else                 { print 'NO';  }

print "\n";

#!/usr/bin/perl

use strict;
use warnings;

if(defined &test){
  print "yes\n";
}
else{
  print "no\n";
}

sub test{
    print "Hallo\n";
}

if (main->can('non_defined_code')) {
  ...
}