Good rule of thumb: whenever you think "lookup table", your next thought should be "use a hash".

my (@list, %find);
@list = qw/a t g c/;
# You can do it manually...
@find{@list} = qw/t a c g/;
# ...or programmatically:
@find{@list} = map {reverse @list[$_*2, $_*2+1] } 0..@list/2-1;

print "$_: $find{$_}\n" for @list;
[download]

Output:

a: t
t: a
g: c
c: g
[download]