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# in package MyModule::Types
declare DBPrefix, as Optional[StrMatch[ qr/ \w+_ /x ]],
...
#    "Optional[StrMatch[(?^x: \w+_ )]]" constrains $_[0] with "StrMatc
+h[(?^x: \w+_ )]" if it exists
#    Value "bar" did not pass type constraint "StrMatch[(?^x: \w+_ )]"
+ (in $_[0])
#    "StrMatch[(?^x: \w+_ )]" is defined as: do { !ref($_) and $_ =~ $
+Types::Standard::_StrMatch{"(?^x: \\w+_ )"} }

state $validate = compile(slurpy Dict[ prefix => DBPrefix ]); #per the
+ docs for named args
my ($param) = $validate->( @_ );
...

# output :
# Reference {"prefix" => "bar"} did not pass type constraint "Dict[pre
+fix=>DBPrefix]" (in $SLURPY) at ./foo.pl line 11