That's how the fat comma works: it turns the word to the left of itself into a string.

$ perl -MO=Deparse,-p -Mstrict -e 'sub b;sub c;(1+b=>c)'
sub b;
use strict;
((1 + 'b'), c());
sub c;
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The behaviour can be sometimes surprising, but it's documented in perlop:

The special quoting behavior ignores precedence, and hence may apply to part of the left operand:

print time.shift => "bbb";
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That example prints something like "1314363215shiftbbb", because the => implicitly quotes the "shift" immediately on its left, ignoring the fact that "time.shift" is the entire left operand.

Update: See also Using constants as hash keys.