The above hash/grep solutions and the link to the Q&A node How can I find the union/difference/intersection of two arrays? are both good, so i won't repeat the solution, but i will answer the other part about why your solution didn't work ..

@ar2 = @ar1[@sl];
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Is an array slice setting elements of @ar2 to elements of @ar1 using the values of @s1 as indices. To write it out longhand, it is doing:

$ar2[0] = $ar1[ $s1[0] ];
$ar2[1] = $ar1[ $s1[1] ];
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Now substitute in the values of @s1 elements:

$ar2[0] = $ar1[ 'b' ];
$ar2[1] = $ar1[ 'd' ];
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And we see why it failed -- the characters 'b' and 'd' cannot be used as the index of an array. Apparently from the output, perl was cast the chars to 0 and that's why both elements are 'a'.

The last lesson here is always use strict and warnings by default .. Adding use warnings (or perl -w) generates these messages:

    Argument "b" isn't numeric in array slice at /tmp/r line 10.
    Argument "d" isn't numeric in array slice at /tmp/r line 10.
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