use Data::Dumper;

my $pn = '5731231234';
my @parts = ( $pn =~ m{ (\d{3}) (\d{3}) (\d{4}) }xms );
print Dumper \@parts;
__END__
$VAR1 = [ 
          '573',
          '123',
          '1234'
        ];
[download]

I'm surprised your database is slow. Maybe it needs an index? For something as simple as this, a DBM::Deep database would be sufficient and probably pretty fast as well.