It reminds me on patern ranges in awk. Could anyone explain what's going under the hood of this code?

There's no need to; perldoc perlop explains it nicely already:

     In scalar context, ".." returns a boolean value.  The
     operator is bistable, like a flip-flop, and emulates the
     line-range (comma) operator of sed, awk, and various
     editors.
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