You almost have your regular expression correct. The following worked as expected for me: 
$example=~s/\w+ing\b/pudding/g;

[download]
I used \w for matching so that any grouping of "word characters" followed by "ing" are replaced.
whoops! duplicated someone else's post :( And thanks to mirod for the "\b" correction.

In reply to Re: Matching 'words' in a string based on a suffix by mr.nick
in thread Matching 'words' in a string based on a suffix by Anonymous Monk

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