Use capturing parens (see perlre):

use warnings;
use strict;

my $s = 'xxx12xxxx345xxxxx6789xxxx';
$s =~ s/([0-9]+)/found$1/g;
print "$s\n";

__END__

xxxfound12xxxxfound345xxxxxfound6789xxxx
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Note the use of + (1 or more matches) instead of * (0 or more).