# note the ?

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

##</code><code>##

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

##</code><code>##

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>.