my $num_subs = `perl -w -0777 -pi -e 'my \$c=s/foo/bar/g;print STDOUT "\$c\\n"' ./foo_test`;
die("Can't execute child: $!\n" if $? == -1'
die("Child killed by signal ".( $? & 0x7F )."\n") if $? & 0x7F;
die("Child exited with error ".( $? >> 8 )."\n") if $? >> 8;

chomp($num_subs);