You can do this with a regex:

my @field = m{ \A (.{20}) \s (.{20}) \s (.{3}) \s (.{3}) \z }msx
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But you shouldn’t. Use unpack instead:

my @field = unpack "A20 x A20 x A3 x A3", $_;
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Not least because the A pack template will automatically trim the whitespace for you. To achieve this using a regex, you have to jump through hoops:

my @field = m{ \A (.{1,20}?) \s+ (.{1,20}?) \s+ (.{1,3}?) \s+ (.{1,3}?
+) \z }msx
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Makeshifts last the longest.