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    function! RunCmd(cmd)
      let fn = expand("%:p")
...

    command! -nargs=1 Run     call RunCmd(<q-args>)
    command!          RunPerl call RunCmd("/usr/bin/perl")

    :RunPerl

    :Run perl

    nnoremap <silent><F5> :<C-u>up|RunPerl<CR>
    inoremap <silent><F5> <C-o>:up|RunPerl<CR>
    vnoremap <silent><F5> <Esc>:up|RunPerl<CR>gv