From the HTML::Template documentation

Q: Why do you use /[Tt]/ instead of /t/i?  It's so ugly!

A: Simple - the case-insensitive match switch is very inefficient.
According to _Mastering_Regular_Expressions_ from O'Reilly Press,
/[Tt]/ is faster and more space efficient than /t/i - by as much as
double against long strings.  //i essentially does a lc() on the
string and keeps a temporary copy in memory.

When this changes, and it is in the 5.6 development series, I will
gladly use //i.  Believe me, I realize [Tt] is hideously ugly.
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I'm sure this is probably mentioned in a few others places, but I remember cause I wrote a patch once for HTML::Template.