#!/usr/bin/perl
use strict;
use warnings;
use Time::Piece;
my $newfilename = userinput2filename($ARGV[0]);

print "got this: $newfilename\n";

sub userinput2filename {
  my $inp = $_[0];
  my $out = $inp;
  while( $out =~ s/`(.+?)`/_userinput2filename($1)/eg ){}
  return $out
}
sub _userinput2filename {
  my $inp = $_[0];
  # date +FORMAT where FORMAT must be understood by POSIX's strftime
  if( $inp =~ /^date\s+\+(.+?)$/ ){ return Time::Piece->new()->strftime($1) }
  # insert other cases you want to substitute
  # eventually there is something in backticks that is not in our list
  die "do not understand '$inp'"
}