use strict;
my $str = ' <text:p text:style-name="P27"><text:span text:style-name="T20">Page #</text:span><text:span text:style-name="T19"><text:tab/></text:span><text:span text:style-name="T20">Content</text:span></text:p>
<text:p text:style-name="P27"><text:span text:style-name="T20">Page #</text:span><text:span text:style-name="T19"><text:tab/></text:span><text:span text:style-name="T20">Content</text:span></text:p>
<text:p>some other text</text:p>
<text:p text:style-name="P27"><text:span text:style-name="T20">Page #</text:span><text:span text:style-name="T19"><text:tab/></text:span><text:span text:style-name="T20">Content</text:span></text:p>';
my $regex = qr/(<.*Page #.*$)/mp;
my $subst = '$1\\nfoo\\nbar\\n';
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