Try something like:

>perl -wMstrict -le
"for my $string (@ARGV) {
   (my $arged = $string) =~ s{ (\d) }{arg$1}xmsg;
   print qq{'$string' yields '$arged'};
   }
"
"1:2"  "-p 1 -t 2 3"  "1 2"  "12"  "no digits"
'1:2' yields 'arg1:arg2'
'-p 1 -t 2 3' yields '-p arg1 -t arg2 arg3'
'1 2' yields 'arg1 arg2'
'12' yields 'arg1arg2'
'no digits' yields 'no digits'
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Update: Extended tests to include "no digits" case.