The simplest method is 
open(PIPE, "| ./unrar e $file");
print(PIPE, $passwd);

[download]

The problem is that the contents of $file are now subject to shell interpretation.

IPC::Open3 (core module) and IPC::Run (better interface) can avoid that problem.

In reply to Re: Simple (hopefully) shell/system() execution question by ikegami
in thread Simple (hopefully) shell/system() execution question by scotchfx

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