Well, that probably means you changed the parameter name from "meta" to
"data" in one place but not in the other place.

<xsl:param name="meta" as="xs:string" required="yes"/>
<xsl:variable name="filename" select="document($meta)" />
<xsl:template match="/">
        <doi>
      <xsl:copy-of select="$filename/doi/text()" />
        </doi>
</xsl:template>