For your own code you can use attributes (experimental feature) to signal what's a method and what's not.

sub foo :method {
  my $self = shift;

  # ...

  return;
}

sub bar {
  # ...

  return;
}

use attributes;
sub is_a_method {
  scalar grep $_ eq 'method', attributes::get($_[0]);
}

for ([ foo => \&foo ], [ bar => \&bar ]) {
  print "$_->[0] is a " . (is_a_method($_->[1]) ? 'method' : 'function
+') . "\n";
}

__END__
foo is a method
bar is a function
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