A variation using index (you didn't say it had to be efficient!)

sub insert_after_first
{
    my ( $a_ref, $afterthis, $insertme ) = @_;
    my $pos = index( join( '', @$a_ref ), $afterthis );
    splice( @$a_ref, $pos+1, 0, $insertme ) if $pos >= 0;
}
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Update: As tall_man mentioned, this only works when the elements of the array are single characters, because the offset for splice represents the number of chars from the beginning of the string/array (rather than the number of individual array elements). Thanks for pointing it out. (I still thought it was a neat idea for the sample data, though!)