The shell used by system is actually given by perl -V:sh. 

$ perl -V:sh
sh='/bin/sh';

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If you want to find out if that's bash, you can use 

$ ls -l /bin/sh
lrwxrwxrwx 1 root root 9 Apr 10 17:08 /bin/sh -> /bin/bash

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Like your code, that only works if and only if /bin/sh is a symlink. A more reliable check is 

$ /bin/sh --version
GNU bash, version 4.1.5(1)-release (x86_64-pc-linux-gnu)
...

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In reply to Re^3: The importance of avoiding the shell by ikegami
in thread The importance of avoiding the shell by jhourcle

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