-i is certainly useful in one-liners, as already mentioned, but it works fine in a full script too. The tricky bit is that -i only works on files that are processed using the magical <>. You might use it like this in your script:

#!perl -i

# or:
# $^I = '';

@ARGV = ('myfile');

while (<>) {
  s/foo/bar/;
  print;
}
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Although that could be reduced to a one-liner, if you wanted: perl -pi -e 's/bar/foo/g' myfile

 

By the way, note that these two code snippets have the same result, but one is cleaner:

if (/foo/) {
  s/foo/bar/g;
}

s/foo/bar/g;
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