Is this what you want?

my ($key, $value);

while(<IN>) {
    if ($_ =~ m/^(interface Vlan([\d]*))/) {
        $key = $1;
        $value = $2;
    }
    if ($_ =~ m/^ ip address \d*\.\d*\.\d*\.\d* \d*\.\d*\.\d*\.\d*/) {
        $hash1{$key} = $value ;
    }
}
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