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open(my $fh, "<+", "your_file") or die("ack: $!");
my @data = <$fh>;
...

truncate $fh, $newsize;
close $fh;

open(my $read,  "<", "your_file") or die("ack: $!");
open(my $write, ">", "tmpfile$$") or die("ack: $!");
...
close $write;

rename $write, $read;