Consider:

use strict;
use warnings;

my $testFile = 'delme.txt';

unlink $testFile;
open my $ioFile, '>', $testFile or die "Failed to open $testFile for u
+pdate: $!\n";

print $ioFile "This is some output text\n";

my $seekBackPoint = tell $ioFile;

print $ioFile "This line will get overwritten (at least partially)\n";
seek $ioFile, $seekBackPoint, 0;
print $ioFile "Overwrite part of a line.\n";

close $ioFile;

open my $iFile, '<', $testFile or die "Failed to open $testFile for re
+ading: $!\n";
print <$iFile>;
close $iFile;
[download]

Prints:

This is some output text
Overwrite part of a line.
ten (at least partially)
[download]

Note: use lexical file handles and the three parameter version of open.