$ perl -E '
    my ($d, $m, $y) = (localtime)[3..5];
    say  $ARGV[0], "_", $d, $m+1, $y+1900, ".log";
' filename
filename_17122019.log

##</code><code>##

$ perl -E '
    my $x = $ARGV[0];
    my ($start, $middle, $end) = $x =~ /^([^`]+)(.+?)([^`]+)$/;
    chomp(my $mid_eval = eval $middle);
    say $start, $mid_eval, $end;
' 'filename_`date +%d%m%Y`.log'
filename_17122019.log