I hope I understand correctly what you are trying to do... does this seem right?

# note the ?

perl -ple 's/<.*?>/<blah>/g' data.txt
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Where data.txt contains:

Hello. I'm <cold>. Very <cold>. I like to be <hot>.
This is <today>. <today> I don't know what happened <yesterday>.
[download]

...and the result:

perl -ple 's/<.*?>/<blah>/g' data.txt

Hello. I'm <blah>. Very <blah>. I like to be <blah>.
This is <blah>. <blah> I don't know what happened <blah>.
[download]

Steve