my @cmd = $check{SHELL};
if (exists $check{SH_OPTS}) {
  push @cmd, ref $check{SH_OPTS} eq 'ARRAY' ? @{$check{SH_OPTS}} : $check{SH_OPTS};
}
push @cmd, $check{CHECK_CODE};
if (exists $check{CODE_OPTS}) {
  push @cmd, ref $check{CODE_OPTS} eq 'ARRAY' ? @{$check{CODE_OPTS}} : $check{CODE_OPTS};
}
open my $fh, '-|', @cmd or die "Can't run check code: $!";
#...