First, start by using use strict; use warnings;. You also have extra quotes in $LFILE = "\@arr10"; use strict; would have caught that.

use strict;
use warnings;

my $LOGFILE12 = '...';

sub GetDecUser {
   my $LFILE = -e $LOGFILE12 ? \@arr12 : \@arr10;
   return grep /declared users/, @$LFILE;
}

print "$_\n" for GetDecUser();
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