diff --git a/crypto/bn/asm/armv4-mont.pl b/crypto/bn/asm/armv4-mont.pl
index 14e0d2d1dd5bcc07d00ae9912e212afced972315..a6fa7cb1bffcc72bb539aef63cc42c6349aeeb05 100644
--- a/crypto/bn/asm/armv4-mont.pl
+++ b/crypto/bn/asm/armv4-mont.pl
@@ -23,6 +23,9 @@
 # than 1/2KB. Windows CE port would be trivial, as it's exclusively
 # about decorations, ABI and instruction syntax are identical.
 
+while (($output=shift) && ($output!~/^\w[\w\-]*\.\w+$/)) {}
+open STDOUT,">$output";
+
 $num="r0";	# starts as num argument, but holds &tp[num-1]
 $ap="r1";
 $bp="r2"; $bi="r2"; $rp="r2";