I expanded it (well, Corion's version) to have multiple lines so the other monks can more easily have a stab at it if they want to.

while (<STDIN>) {
    if(/^(\d{4}-\d{2}-\d{2}\s\d{2}:\d{2}:\d{2}),(\d{3})/) {
        $o="$1$2";
        @a = reverse split( "[-: ]", $1);
        $a[3]-=1;
        $a[4]-=1;
        $a[5]-=1900;
        if(!$l){    # does this ever happen?
            $t=localtime(@a,0,0,0)*1000+$2;
            $l=1;
        } else {
            $t2=localtime(@a,0,0,0)*1000+$2;
            $e=$t2-$t;
            $t=$t2;
            $l=0;
            say "$o $e";
        }
    }
}
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I don't think the first if() branch is ever executed.