I usually don't initialize variables when I cannot give them a usefull value. But sometimes you have to. For example a procedure to remove duplicates in a file sorted on the duplicate key.

my $old_key = "";
while (<>) {
     my $key = (split /;/, $_)[2]
     next if $key eq $old_key;
     # ...
}
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I need to initialize $old_key to prevent a warning when reading the first line of the file.