enclosure.xsl

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:atom="http://www.w3.org/2005/Atom">

  <xsl:template match="atom:entry">
    <xsl:for-each select="atom:link[@rel='enclosure']">
      <xsl:apply-templates select=".." mode="copy">
        <xsl:with-param name="enclosure" select="position()"/>
      </xsl:apply-templates>
    </xsl:for-each>
  </xsl:template>

  <xsl:template match="atom:entry" mode="copy">
    <xsl:param name="enclosure"/>
    <xsl:copy>
      <xsl:apply-templates select="@*|node()"/>
      <xsl:copy-of select="atom:link[@rel='enclosure'][$enclosure]"/>
    </xsl:copy>
  </xsl:template>

  <xsl:template match="atom:link[@rel='enclosure']"/>

  <!-- Identity template -->
  <xsl:template match="@*|node()">
    <xsl:copy>
      <xsl:apply-templates select="@*|node()"/>
    </xsl:copy>
  </xsl:template>
</xsl:stylesheet>

source.pxsl

<<<?xml version="1.0" encoding="UTF-8"?>>>

xsl:stylesheet -version=1.0 -xmlns:xsl=http://www.w3.org/1999/XSL/Transform -xmlns:atom=http://www.w3.org/2005/Atom

  template atom:entry
    for-each atom:link[@rel='enclosure']
      apply-templates .. copy
        with-param enclosure position()

  template atom:entry -mode=copy
    param enclosure
    copy
      apply-templates @*|node()
      copy-of atom:link[@rel='enclosure'][$enclosure]

  template atom:link[@rel='enclosure']

  -- Identity template
  template @*|node()
    copy
      apply-templates @*|node()