First, I would advise you check the result of the open i.e.:

open($self->{FILE}, ">>$self->{file_path}$self->{file_name}") || die "
+Could not open file $!\n";
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That, however, does not solve your problem. I think that perl is getting confused by the ->. This works:

my $self = {};

open($self->{FILE}, ">> text.txt") || die "Could not open file : $!\n"
+;
my $fh = $self->{FILE};
print $fh "some text here\n";
close($self->{FILE});
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