'Can't use an undefined value as filehandle reference' error in Perl

Question

I am getting this error when I run my perl script:

Can't use an undefined value as filehandle reference at scr line 44.

Line 44:

open my $fh, "|-", "mail", "-s", $subject, $owner, "-c", $sendto
    or die "$0: could not start mail: $!";

Everything looks okay. What does the error mean?

Do you have an ancient version of perl, such as 5.005 or so, installed? — mu is too short
– mu is too short, Commented Nov 17, 2010 at 7:42
Why are you working with a nine year old version of Perl? Update your Perl, your sysadmin or your job :-) — Dave Cross
– Dave Cross, Commented Nov 17, 2010 at 12:07

Michael Carman · Accepted Answer · 2010-11-17 16:35:32Z

4

I think the error message is in error. This code:

open my $fh, '|-', 'perl', '-v' or die "$!\n";

dies with the error message

Can't use an undefined value as filehandle reference...

under Perl 5.6.1, but dies with

List form of pipe open not implemented...

under Perl 5.12.1 (on Win32).

"List form" means specifying the program and its arguments as separate values. The single-value form runs on both versions:

open my $fh, '|-', 'perl -v' or die "$!\n";

edited Nov 17, 2010 at 16:35

answered Nov 17, 2010 at 15:52

Michael Carman

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mu is too short · Accepted Answer · 2010-11-17 08:13:49Z

1

That's a pretty old perl, I'm guessing that that version doesn't support lexical file handles so "open my $fh" won't work as ancient perl will see that as, essentially, open undef and hence the error message. Possible solutions:

Upgrade your perl out of the bronze age.
Use the old school open FH syntax instead.
Use gensym to generate a symbol that you can use as a file handle.
Use one of the IO::* classes of plain open.

answered Nov 17, 2010 at 8:13

mu is too short

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2 Comments

tchrist Over a year ago

It supports lexical filehandles: my $fh = "STDIN" is of course a lexical filehandle; so is my $fh = do{local *FH}. It simply doesn’t support the autovivification of lvaluable arguments previously undefined into references to typeglobs blessed into the IO::Handle package with a destructor to close and hide any failure on said handle — which is a different thing.

mu is too short Over a year ago

So I'm thinking around the right area then: upgrade perl or downgrade the code. Been awhile since I've dealt with 5.6 and I switched to IO::Handle as soon I found out about it.

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