how to test if BLAST installed in perl script?

Hi, eveyone,

In a perl script, I want to test if blast was installed using the following lines. However on a computer without BLAST, there is no error message shown. What's wrong with the script?  Thank you very much!

check_blast();sub check_blast {    (open (my $BLAST, "blastn -h |")) || print STDERR "ERROR: blast not installed";    close $BLAST;}