if statements test a control statement to see whether it is true or false. else and elsif clauses can also be tagged on the back.

#your basic if
if($value==1){
    print "value is equal to 1\n";
}

#your basic if/else
if($value==1){
   print "value is equal to 1\n";
} else{
   print "value is not equal to 1\n";
}

#your basic if/elsif/else

if($value==1){
   print "value is equal to 1\n";
} elsif($value==2){
   print "value is equal to 2\n";
} elsif($value==3){
   print "value is equal to 3\n"; 
} else{
   print "value is not equal to 1,2, or 3\n";
}
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Unless statements offer you the opposite functionality. If the control statement evaluates to false then the statements inside the block are executed.