Create an hash of department => IO::File object for that department's log. Read through the main log, determinining what department the entry is for and printing to the corresponding object.

my %handles;
for my $dept ( @departments ) {
  $handles{ $dept } = IO::File->new;
  $handles{ $dept }->open( "> $logdir/$dept.log" ) or die "Open failed
+: $!\n";
}

while( <INLOG> ) {
  my $dept = divine_department( $_ );
  $handles{ $dept }->print( $_ );
}
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