I occasionally like to remove whole-line comments from config files or Perl scripts (esp. when the comments prevent me from seeing the code clearly). This works great:

perl -pe 'next if /^(\s*)#/' file.conf > clean-file.conf"
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Remove the (\s*) if your definition of "whole-line comment" doesn't include whitespace before the '#'. Also note that I deliberately avoid in-place edit, as the point is to make a clean copy.

On Windows, I often like to backup generated files before doing a second test run. Since I want to compare both sets of output, sometimes I need to leave the extension intact:

perl -e "for(glob '*.xls'){$o=$_; s/(\.xls)$/.old$1/i;rename $o,$_}"
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works smashingly (specifically for XLS files, that one).

And finally, also on Windows (where find means something different than on UNIX), I sometimes want to remove all files in a directory (not the whole tree) that are more than 48 hours old:

perl -e "for(glob '*'){next unless -f;@s=stat;unlink($_) if (time-$s[9
+])>60*60*48;}"
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Update: And to search the whole tree:

perl -MFile::Find -e "find(sub{@s=stat;unlink if (time-$s[9])>60*60*48
+ && -f},'.')";
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