Perl Module Method Calls: Can't call method “X” on an undefined value at ${SOMEFILE} line ${SOMELINE}

Question

All over the place, especially in DBI, I see this message come up all the time. It\'s confusing, because the first thing that comes to mind is that the arguments I\'m passin

Answer 1

You're conflating several different ways to handle modules and objects - and ending up with one that doesn't work.

Here are four approaches that do work:

1/ My::Module is a library. trim is not exported.

$ cat My/Module.pm 
package My::Module;

use strict;
use warnings;

sub trim {
  my $str = shift;

  $str =~ s{ \A \s+ }{}xms; # remove space from front of string
  $str =~ s{ \s+ \z }{}xms; # remove space from end of string
  return $str;
}

1;
$ cat test 
#!/usr/bin/perl

use strict;
use warnings;

use My::Module;

# Note: No $ and :: not ->
print My::Module::trim( " \t hello world\t \t" );

2/ My::Module is a library. trim is exported.

$ cat My/Module.pm 
package My::Module;

use strict;
use warnings;

use Exporter;
our @ISA    = qw(Exporter);
our @EXPORT = qw(trim);

sub trim {
  my $str = shift;

  $str =~ s{ \A \s+ }{}xms; # remove space from front of string
  $str =~ s{ \s+ \z }{}xms; # remove space from end of string
  return $str;
}

1;
$ cat test 
#!/usr/bin/perl

use strict;
use warnings;

use My::Module;

print trim( " \t hello world\t \t" );

3/ MyModule is a class. trim is a class method.

$ cat My/Module.pm 
package My::Module;

use strict;
use warnings;

sub trim {
  # Note class name passed as first argument
  my $class = shift;
  my $str = shift;

  $str =~ s{ \A \s+ }{}xms; # remove space from front of string
  $str =~ s{ \s+ \z }{}xms; # remove space from end of string
  return $str;
}

1;
$ cat test 
#!/usr/bin/perl

use strict;
use warnings;

use My::Module;

# Note: Not $ and -> not ::
print My::Module->trim( " \t hello world\t \t" );

4/ MyModule is a class, trim is an object method.

$ cat My/Module.pm 
package My::Module;

use strict;
use warnings;

# Need a constructor (but this one does nothing useful)
sub new {
  my $class = shift;

  return bless {}, $class;
}

sub trim {
  # Note: Object method is passed an object (which is ignored here)
  my $self = shift;
  my $str = shift;

  $str =~ s{ \A \s+ }{}xms; # remove space from front of string
  $str =~ s{ \s+ \z }{}xms; # remove space from end of string
  return $str;
}

1;
$ cat test 
#!/usr/bin/perl

use strict;
use warnings;

use My::Module;

my $trimmer = My::Module->new;

print $trimmer->trim( " \t hello world\t \t" );

I think that you were trying for option 1. In this case, I think I'd recommend option 2.

And to answer your final question. You are getting that error because you are trying to call a method on a variable ($My::Module) which is undefined.