Do the separating in Perl (for example with the split statement) and pass the individual arguments/parameters to the XSLT. So something like:

In the xslt:

<xsl:param name="param_1"/>
<xsl:param name="param_2"/>
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In the Perl script:

my ($par1, $par2) = split (/ /,$str);

my $results = $stylesheet->transform($source,
        XML::LibXSLT::xpath_to_string(param_1 => $par1),
        XML::LibXSLT::xpath_to_string(param_2 => $par2)
);
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NB The utility function xpath_to_string takes care of the quoting. This is probably the better/easier way of doing things. See the documentation for details. See playing with XSLT for a bigger example.