use strict;

my @arr = qw(a b c d c e f);
insert_after_first(\@arr, "c", "x");

sub insert_after_first
{
    my $arr = shift;
    my $before = shift;
    my @new = @_;

    @$arr = map { 
        defined $before && $_ eq $before ? 
            do { $before = undef; @new } :
            $_
    } @$arr
}

use Data::Dumper;
print Dumper(\@arr);