I'd include the dot \. into the capture,

$langhash{'dígito verificador incorrecto.'} = 'digit incorrect.';
my $string = '(5/4/07 4:24:03 PM CDT) 1473721:   ##   dígito verificad
+or incorrecto. ##pp';  
if( $string =~ s/##\s*([^#]+\.)\s*##/##$langhash{$1}##/ ){  
   print "|$langhash{$1}|\n";
   print "|$1|\n";
   print $string;
}
[download]

That will do it.

Regards,

M.