How can I compare a number against a range in bash or Perl?

It's even better in Perl6.

Chained comparison operators:

if( 2 <= $x <= 5 ){
}

Smart-match operator:

if( $x ~~ 2..5 ){
}

Junctions:

if( $x ~~ any 2..5 ){
}

Given / When operators:

given( $x ){
  when 2..5 {
  }
  when 6..10 {
  }
  default{
  }
}