Several things are wrong. /0-9/ will match '0-9' (exactly). And since it's a negated match, basically you are accepting all input unless it contains the literal text '0-9'.

You need a character class.

if( $sid =~ /^[^0-9]$/ ) {
    # exit here......
}
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The above code states if there are any characters in the string that are not numbers 0 through 9, exit.

It might actually be better to do it this way though:

if( $sid !~ m/^([0-9]{1,5})$/ ) {
    # exit
} else {
    $input = $1;
}
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...or better yet...

if( $sid =~ m/^([0-9]{1,5})$/ ) {
    $input = $1;
} else {
    # exit
}
[download]

I think negated matches ( the !~ operator ) are usually just a way of adding confusion, and should be avoided unless absolutely necessary.