# in package MyModule::Types
declare DBPrefix, as Optional[StrMatch[ qr/ \w+_ /x ]],
  message { 'The table prefix must end in an underscore. ' };

# in package MyModule
use Type::Params qw/ compile /;
use My::Types qw/ DBPrefix /; 

state $validate = compile( DBPrefix );
my ($param) = $validate->( @_ );

# in foo.pl
$obj->method( 'bar' );

# output:
# The table prefix must end in an underscore.  (in $_[0]) at ./foo.pl line 11
#    "DBPrefix" is a subtype of "Optional[StrMatch[(?^x: \w+_ )]]"
#    Value "bar" did not pass type constraint "Optional[StrMatch[(?^x: \w+_ )]]" (in $_[0])
#    $_[0] exists
#    "Optional[StrMatch[(?^x: \w+_ )]]" constrains $_[0] with "StrMatch[(?^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+_ )"} }

##</code><code>##

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

# in foo.pl
$obj->method( 'bar' );

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