I think you mean the first 10 digits are the phone number, not the first 7.

If you can assume that the first 10 digits are the phone number, and the remaining digits are the extension (which is a big assumption):

# Get rid of all non-digits
$number =~ s/\D//g;

# Break the number into groups of digits
my $phone = substr($number,0,10);
my $extension = substr($number,10);
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buckaduck