For this sort of task I reach for mod?;;HTML::TreeBuilder. Consider:

use strict;
use warnings;
use HTML::TreeBuilder;

my $html = <<HTML;
<p>June <b>25, </b>1998</p>
<p>November 12,2006 September 21, 1999</p>
<p>December 36,
10</p>
HTML

my $tree = HTML::TreeBuilder->new_from_content ($html);

for ($tree->look_down ('_tag', 'p')) {
    my $text = $_->as_text ();
    print "$1\n" while $text =~ /(\w+\s+\d+,\s*\d+)/g;
}
[download]

Prints:

June 25, 1998
November 12,2006
September 21, 1999
December 36, 10
[download]