use strict;
use warnings;

my @list = (1,2,3,4,5,6,7,8,9,8,7,6,5,4,3,2,1);

my @indexes;

### Method 1: two passes over the list ###

# Find all the fives
for my $idx (0 .. $#list) {
    push @indexes, $idx if $list[$idx] == 5;
}

# For each five we find, show it (position and value) and
# the three following values
for my $idx (@indexes) {
    print "list[$idx]=$list[$idx]: ";

    for my $t (1 .. 3) {
        print $list[$idx + $t], " ";
    }
    print "\n";
}

### Method 2: one pass over the list ###

for my $idx (0 .. $#list) {
    # Skip printing unless we're at a five
    next unless $list[$idx] == 5;

    print "list[$idx]=$list[$idx]: ",
          # use a list slice to get the elements
          join(", ", @list[$idx+1 .. $idx+3]),
          "\n";
}