the letters are hex.

Huh?

So if you have the hex number 7A, you want to retain the 7 and discard the A? That will cause the number 7A, which is 122 in decimal, to become 7 in decimal. Is this really what you want? What you're doing is simply discarding all digits of value greater than nine. ...doesn't make sense to me.

Nevertheless, here's another way to do it..:

$string =~ tr/0-9.//cd;

The /c flag means to complement the characters enumerated on the left side of the tr/// operator. And /d means to delete any character from the target string that is found in the list. /cd means delete any character not found in the list on the lefthand side.

#perl -e '$_="1aa23.4a";$_=~s/[a-f]//gi;print "$_\n"'
123.4
#
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