use strict;
my $str = '<a href="http://www.sample.ru/docs/price.pdf" alt="цены">прайс</a>
<a href="https://public.library.ru/menu.pdf" data-order="52">менюшечка</a>
<a href="https://yandex.ru/web/shtrafy.pdf">штрафы</a>';
my $regex = qr/href=("|')(.+?)("|')/p;
my $subst = 'href=$1docs.google.com/viewer?url=$2$1';
my $result = $str =~ s/$regex/$subst/rg;
print "The result of the substitution is' $result\n";
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for Perl, please visit: http://perldoc.perl.org/perlre.html