my ($line_in) = "\n<?xml-stylesheet href=\"perl1.css\" type=\"text/css\"?><link href=\"//www.perl.org/css/perl1.css\" rel=\"stylesheet\"><link href=\"/css/perl.css\" rel=\"stylesheet\">  <?xml-stylesheet href=\"perl1.css\" type=\"text/css\"?>\n\n";

my @ss = $line_in =~ /<(?=[^>]*stylesheet).*?href *= *"*([^">]+)"/gis;
print "$_\n" for @ss;

Prints:
perl1.css
//www.perl.org/css/perl1.css
/css/perl.css
perl1.css