If your relevant data is being held at a constant size (like most time strings of that type are), you can probably do something like this:

$_ = substr($_, 0, 24) for @array;
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That's probably going to be more efficient than most any regexp. If you cannot guarantee that you want characters 0..24 always from this string, or that the data format will change, you should probably go with a slightly more expensive regexp alternative, as posted by others.