Argh! We're caching the results of the checks for zip & perl from way back when so we need to check them against : as an error condition.

configure

@@ -2757,7 +2757,7 @@ fi
 test -n "$PERL" && break
 done
 
-if test -z "$PERL"; then
+if test -z "$PERL" || test "$PERL" = ":"; then
     { echo "configure: error: perl not found in \$PATH" 1>&2; exit 1; }
 fi
 
@@ -2931,7 +2931,7 @@ fi
 test -n "$ZIP" && break
 done
 
-if test -z "$ZIP"; then
+if test -z "$ZIP" || test "$ZIP" = ":"; then
     echo "configure: warning: zip not found in \$PATH. Disabling jar packaging" 1>&2
     MOZ_DISABLE_JAR_PACKAGING=1
 fi

configure.in

@@ -290,7 +290,7 @@ AC_PROG_LN_S
 AC_PROG_AWK
 AC_PATH_PROGS(EMACS, xemacs lemacs emacs, :)
 AC_PATH_PROGS(PERL, perl5 perl )
-if test -z "$PERL"; then
+if test -z "$PERL" || test "$PERL" = ":"; then
     AC_MSG_ERROR([perl not found in \$PATH])
 fi
 
@@ -318,7 +318,7 @@ AC_PATH_PROG(WHOAMI, whoami, :)
 AC_PATH_PROG(AUTOCONF, autoconf, :)
 AC_PATH_PROG(UNZIP, unzip, :)
 AC_PATH_PROGS(ZIP, zip)
-if test -z "$ZIP"; then
+if test -z "$ZIP" || test "$ZIP" = ":"; then
     AC_MSG_WARN([zip not found in \$PATH. Disabling jar packaging])
     MOZ_DISABLE_JAR_PACKAGING=1
 fi