The two easiest ways to pass information to another process when invoking it is on the command line, and through standard input.

Say you call the second program from the shell, not the first one. How does it expect the input? It's probably one of these two:

second_program filename            # as a command line option
echo "filename" | second_program   # piped into its standard input
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When calling second_program, you need to do one of these:

system "./second_program", $filename and die ... # pass it on the comm
+and 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)
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