I am testing a function in one of my modules which utilizes qx// to do it's work. I'd like to verify exactly how it's calling qx// from within my test script by overriding qx// and capturing the arguments. I've done this by overriding CORE::GLOBAL::readpipe, which according to Programming Perl 3nd ed, pg 772, is the internal function that implements the qx// operator.

The problem is that the variable given to qx// as an argument is not being expanded in my test script. Here is the sample code. Foo.pm is the module that uses the qx// operator. test.pl is my test script which showcases the problem.

Foo.pm:

package Foo;
use strict;

sub pdftotext {
    my $self = shift;
    my $args   = join ' ', @_;
    my $output = qx{/usr/bin/pdftotext $args};
}

1;
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test.pl:

# Override the qx// operator to capture args to $qx_args
my $qx_args;
BEGIN { *CORE::GLOBAL::readpipe = sub { $qx_args = $_[0] } }

# Load module that uses qx//
use Foo;

# Call method which will use our overridden qx// function
Foo->pdftotext('/tmp/myfile.pdf');

# print arguments to qx//
print "qx// args were: [$qx_args]\n";
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This prints the following output:
qx// args were: [/usr/bin/pdftotext $args]

As you can see, $args is not being expanded. I've verified that qx// is evaluated in a double-quote interpolative context, unless the delimiters used are single-quotes, which they are not in this case.

I believe the problem lies somewhere in the anonymous sub that I'm using to override readpipe. My question is how can I modify that sub to properly expand the variable $args?

I am also open to a better way of doing this from my test script if my approach is wrong. I could not find a Test:: module on CPAN that overrides qx// or readline.