second_program filename            # as a command line option
echo "filename" | second_program   # piped into its standard input

##</code><code>##

system "./second_program", $filename and die ... # pass it on the command line
                                                 # note, system needs "and" for error checking

open my $fh, "| ./second_program"    or  die ... # via stdin
print $fh $filename;                             # (note, no comma on this line)