my @fields = unpack('A34 A30 A40 ...', $string);
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my @field_sizes = (34, 40, 40, ...);
my $rec_format = join '', map "A$_", @field_sizes;

my @fields = unpack($rec_format, $string);
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Then use $fields[0] instead of $FIELD01, $fields[1] instead of $FIELD02, etc.