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

$ perl -E '
    my $x = $ARGV[0];
...
    say $start, $mid_eval, $end;
' 'filename_`date +%d%m%Y`.log'
filename_17122019.log