If elegance is more important than effeciency, try this recursive version:

sub insert_after_first {
    return if @_ < 3;
    my ( $elem, $new, $front ) = splice @_, 0, 3;
    return ( $front, $new, @_ ) if $front eq $elem;
    return ( $front, insert_after_first( $elem, $new, @_ ) );
}
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Otherwise, I'd say just keep the version that you've got, although I'd change the loop initialization to for (0 .. $#arr), like ikegami shows above.