The syntax error you're getting is accurate:

You cannot do a double-comparison as a single expression. The answer is to use the logical and operator to chain two expressions.

if ( $X > 9 and $X < 100 ) {
    $Y = $X + 1900;
} elsif ( $X >= 0 and $X < 10 ) {
    $Y = $X + 2000;
}
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By the way, that's just one way to do it.