In line 1 below, the parens around $a1 force a scalar context.

Actually, that's list context.

What you're seeing here is how the comma operator behaves in list and scalar context. See perlop - Comma Operator for details.

As a further example, compare scalar context (as in your example):

$ perl -Mstrict -Mwarnings -E 'my %a2 = ( x=>4, y=> scalar (5,7,9) ); 
+say $a2{y}'
Useless use of a constant (5) in void context at -e line 1.
Useless use of a constant (7) in void context at -e line 1.
9
[download]

with list context (removing scalar from your code):

$ perl -Mstrict -Mwarnings -E 'my %a2 = ( x=>4, y=> (5,7,9) ); say $a2
+{y}'
5
[download]

Update: Actually, a better example (for the list context) would be:

$ perl -Mstrict -Mwarnings -E 'my %a2 = ( x=>4, y=> (5,7,9) ); say $a2
+{y}; say $a2{7}'
5
9
[download]

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