The docs for Time::Piece explain it thus (emphasis mine):

The months and years can be negative for subtractions. Note that there is some "strange" behaviour when adding and subtracting months at the ends of months. Generally when the resulting month is shorter than the starting month then the number of overlap days is added. For example subtracting a month from 2008-03-31 will not result in 2008-02-31 as this is an impossible date. Instead you will get 2008-03-02. This appears to be consistent with other date manipulation tools.

In reply to Re^2: How to get last month date using Date::Calc (caution advised) by hippo
in thread How to get last month date using Date::Calc by kanewilliam7777

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