# 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";