Hello deep27ak, and welcome to the Monastery!

push(my @array, $lanip) if ! $seen{$lanip}++;
print "@array\n";
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The logic is wrong. This creates a new @array on each iteration of the foreach loop, conditionally populates it with a single element, and then prints it out whether it contains the element or not. You need something like this (untested):

my @array;

foreach my $server (@{$servers->{server}})
{
    my $node  = $server->{Node}    . "\n";
    my $lanip = $server->{LanIP}   . "\n";
    my $mask  = $server->{Netmask} . "\n";

    substr($lanip, 11, 3) = "0";
    push(@array, $lanip) if ! $seen{$lanip}++;
}

print "@array\n";
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Note also that use of the XML::Simple module in new code is discouraged — see STATUS-OF-THIS-MODULE of XML::Simple.

Hope that helps,