In scalar context? A false scalar.

In list context? Nothing. It returns zero scalars.

a => 'a' =~ m/b/,
b => 'asdf'
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is just a fancy way of writing

'a', 'a' =~ m/b/, 'b', 'asdf'
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The expression within the hash constructor ({}) is evaluated in list context. Since the above list was evaluated in list context, it's individual elements were evaluated in list context. On a failed match, the above is therefore equivalent to

'a', 'b', 'asdf'
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Solutions:

a => scalar( 'a' =~ m/b/ ),
b => 'asdf'
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a => !!( 'a' =~ m/b/ ),
b => 'asdf'
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a => 'a' =~ m/b/ ? 1 : 0,
b => 'asdf'
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