Because the plus sign is a regular expression metacharacter and must be escaped.

$ perl -E '
> $str = q{DSI+DSI+};
> say q{Yes} if $str =~ m{(DSI\+DSI)};
> say qq{Matched $1, length @{ [ length $1 ] }};'
Yes
Matched DSI+DSI, length 7
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I hope this is helpful.