A major problem with your code are these lines:

    if ($neighbor eq undef){}
    elsif ($vcid eq undef) {}
    else {
[download]

undef can NOT be used in a comparison, you have to use the defined function to determine if a variable is defined or not.

You also don't need the if/elsif/else structure:

    if ( !defined $neighbor && !defined $vcid ) {
[download]

Another problem is a lot of "uninitialized value" messages which can be fixed by initializing all your variables.

my $vcid;
my $tag;
my @splittag;
my $pop;
my $xcon;
my $cos;
my @splitcos;

# change those to:

my $vcid = '';
my $tag = '';
my @splittag;
my $pop = '';
my $xcon = '';
my $cos = '';
my @splitcos;
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    $unit = $splitunit[2];

# change to:

    $unit = $splitunit[2] || '';
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    $vlanida = $splitvlan[2];
    $vlanidb = $splitvlan[4];

# change to:

    $vlanida = $splitvlan[2] || '';
    $vlanidb = $splitvlan[4] || '';
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        $cos = $splitcos[2];

# change to:

        $cos = $splitcos[2];
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    $neighbor = $splitxc[1];
    $vcid = $splitxc[2];

# change to:

    $neighbor = $splitxc[1] || '';
    $vcid = $splitxc[2] || '';
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    $pop = $splittag[4];

# change to:

    $pop = $splittag[4] || '';
[download]

    if ($neighbor eq undef){}
    elsif ($vcid eq undef) {}
    else {

# change to:

    if ( length $neighbor && length $vcid ) {
[download]

Try running your code with those changes and post again with your feedback.