According to [1], the epitaph of conde don Peranzules starts with the following (emphasis mine):
Aquí iaze sepultado
un conde diño de fama,
un varón muy señalado,
leal, devoto, esforeado.
Don Perancurez se llama.
El qual sacó de Toledo
de poder del rey pagano
al rey que con pena y miedo
tuvo siempre el brazo quedo
al horadar de la mano.
The same source [1] suggests (page 86) that "al horadar de la mano" is equivalent to "al horadarle la mano" (and appears that way in other sources). On page 83, it says
[...] tuvo siempre el brazo quedo, o sea quieto e inmóvil, al horadarle la mano, o ante la amenaza de horadársela proferida en alta voz por Al-Mámun, como explica Duran [...].
i. e., the meaning is that the king always held his arm still, even after hearing Al-Mámun threaten to drill a hole in his hand.
As suggested on page 86, José Zorrilla's lines
tuvo siempre el brazo quedo
las palmas al horadarle.
seem to refer to the same event. The phrase "las palmas al horadarle" seems to have an unusual word order, and the two lines could be interpreted as "tuvo siempre el brazo quedo / al horadarle las palmas". This would have the same meaning: the king held his arm still when faced with the threat of his palms being drilled through.
This is a very unusual word order and probably only makes sense in poetry. Note though that the Spanish language allows plenty of freedom when it comes to word order in sentences.
[1] JOSÉ ZURITA NIETO, "APUNTES DOCUMENTADOS SOBRE EL AÑO DE LA MUERTE DEL CONDE DON PEDRO ASSUREZ Y ACERCA DE su sepultura, epitafio y aniversario en la S. I. M. de Valladolid", available at https://bibliotecadigital.jcyl.es/es/catalogo_imagenes/grupo.cmd?path=10067861
