The command line switch -i does exactly this, but is usually used for command-line short cuts.

Update: For example, you could do exactly what you specify in your post with the one liner:

perl -pi -e 's/^((?:[^:]*:){4})/${1}UNDEF:/ if /^test:/' file.txt

However, if this script is intended to do more than just a simple search-and-replace, you'll likely be better off writing a proper script where you explicitly output a new file.

In reply to Re: Replace current line in while loop by kennethk
in thread Replace current line in while loop by walkingthecow

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