The p and the i are seperate options. You definitely want to keep the p, and you definitely want to keep the i ("inplace"), but if you don't specify an extention, the backup file isn't created. This is all documented in perlrun. 
perl -pi -e 's/apple/orange/gi'

[download]

Caveat: i without an extention does not work in Windows.

In reply to Re^3: String substitution by ikegami
in thread String substitution by kirk123

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