Because you've switched BOIN and BOOUT.

You need to write to BOOUT and read from BOIN--you were doing the opposite. It should read:

    #!/usr/bin/perl -w
    use strict;

    use IPC::Open2;

    print open2(\*BOIN, \*BOOUT, "cat"); 

    print BOOUT "proclist\n";
    print BOOUT "quit\n";
    close BOOUT;

    while (<BOIN>) {
        print;
    }
    close BOIN;
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If you'd had warnings on in your original, you would have received this warning:

    Filehandle main::BOIN opened only for input at foo.pl line 8.
    Filehandle main::BOIN opened only for input at foo.pl line 9.
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By the way, as perlipc warns, watch out for Unix buffering. In this case (using cat) you might want to use the -u flag for unbuffered output. In your case, it doesn't matter much, since you close the output filehandle. But in general, it's something to think about.