There is a module, Convert::IBM390, that supports various conversion functions for data to/from a mainframe. One is packeb:

use Convert::IBM390 qw(packeb);
my $comp3 = packeb( 'p4', 240 );  #77 A  PIC S9(7) COMP-3.
print $comp3;
__END__
~~output~~
0x0000240c  (of course, in binary)
[download]

It works well, and handles negative values.

Good luck. -c