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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In reply to Re: What does a failed regular expression match actually return? by ikegami
in thread What does a failed regular expression match actually return? by Anonymous Monk

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