Fine, you requested an easy method; although, faster methods could be implementd with map or grep, assuming we don't have detailed knowledge if elements are already sorted in the array. Here is on easy method:

my @arr = 1..20 ;
my $num = 4; 
my %h ;

@h{@arr} =  (undef) x @arr;
exists $h{$num}  and  print qq(Yes, $num exists inside \@arr)
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