#
#  Here's the stylesheet.  The annoying part is that the
#  actual change is three lines, the rest is grammar.  You
#  can add multiple templates to make simultaneous changes.
#
my $xsl = <<EOS;
<?xml version="1.0"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0">
 <xsl:output method="xml" indent="yes"/> 

 <xsl:template match="/">
    <transform>
       <xsl:apply-templates/>
    </transform>
 </xsl:template>

 <xsl:template match="//time">
   $new_time
 </xsl:template> 

</xsl:stylesheet>
EOS

#
# The perl is down here
#
use XML::XSLT;

my $xslt = XML::XSLT->new ($xsl, warnings => 1);
$xslt->transform ($xmlfile);
print $xslt->toString;
$xslt->dispose();