This does what you ask:

#!/usr/bin/perl

my @fred = ( "a", "b", "c" );

splice @fred, 2, 0, "hello";

print "@fred\n";
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splice() takes four arguments: the array, the position to start at, the length of the sub-list to replace, and the list to replace it with. That sub-list length is the tricky part: make it 0 to insert things into the array. Any ordinal number "overwrites" that many elements in the array. For instance, make the third argument 1 and you'll see the "c" disappear since it's replaced.