You could also alias vi to run a script that will execute vi, then chmod the file if appropriate. Here's a start:

#!/bin/sh

cmdline=""
while [ $# -gt 0 ]; do
  case "$1" in
    --) break;;
    [+-]*) cmdline="$cmdline $1"; shift;;
     *) break;;
  esac
done

fn=""
if [ $# -gt 0 ]; then
  fn=$1
fi

vi $cmdline $fn
ec=$?
if [ -n "$fn" -a -f "$fn" ]; then
  case "$fn" in
    *.pl) chmod +x "$fn";;
  esac
fi

exit $ec
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