An alternative to using a regular expression with a capture of a digit and subsequent replacment with args$1 would be to use a zero-width look-ahead, effectively inserting "args" at any point followed by a digit.

use strict;
use warnings;

my @inputs = (
   q{A) 1:2},
   q{B) -p 1 -t 2 3},
   q{C) 1 2},
   q{D) 12},
   );

my @outputs =
   map { s{(?=\d)}{args}g; $_ }
   @inputs;

print qq{$_\n} for @outputs;
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The output.

A) args1:args2
B) -p args1 -t args2 args3
C) args1 args2
D) args1args2
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I hope this is useful.

Cheers,

JohnGG