Here is how I would do it:

my $file = "/foo/bar.txt";
open(FOO, "+< $file") or die qq[Could not open "$file": $!\n];
@slurp = <FOO>;
seek(FOO, 0, 0);
print FOO "I am at the very start of the file!\n";
print FOO @slurp;
truncate(FOO, tell(FOO));
close(FOO);
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