@a = (12, 18, 56, 76, 99);
$in = <STDIN>;

for(0..$#a){
   die "found at $_\n" if $in == $a[$_];
}
print "not found\n";
[download]

or even prettier:

@a = (12, 18, 56, 76, 99);
$in = <STDIN>;
sub{die "found at $_\n" if $in == $a[$_]}->() for(0..$#a);
print "not found\n";
[download]

or

@a = (12, 18, 56, 76, 99);
$in = <STDIN>;
for(0..$#a){
   $i = $_ if $in == $a[$_];
}
$i == 0 ? print "not found\n" : print "found at $i\n";
[download]