<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">

<xsl:variable name="var" select="document('xxx.xml')/xxxx/xxxx"/>

    <xsl:template match="tag1">
        <tag1>
	    <xsl:attribute name="attr1">
	        <xsl:value-of select="$var" />
	    </xsl:attribute>
        </tag1>
    </xsl:template>

</xsl:stylesheet>

use XML::LibXSLT;
use XML::LibXML;

my ($style_file, @source_files) = @ARGV;

my $parser = XML::LibXML->new();
my $xslt = XML::LibXSLT->new();
my $stylesheet = $xslt->parse_stylesheet_file($style_file);

foreach my $source_file (@source_files) {
    my $source_doc = $parser->parse_file($source_file);
    my $result = $stylesheet->transform($source_doc);
    print $stylesheet->output_string($result);
}