A failed match doesn't reset $1 and $2. You need to guard your assignments with if statements:

my( $tag1, $tag2 );

# get tag1 value
if( $data =~ m/tag1=(\d+)/) {
    $tag1 = $1;
};

# get tag2 value
if( $data =~ m/tag2=(\d+)/ ) {
    $tag2 = $1;
};
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Also note that the /g modifier doesn't make sense.