Perl - Conditional statements

if (expression1

)
{

statement(s) executed if expression1 is true;

}
elsif (

expression2

)
{

statement(s) executed if expression1 is false, but expression2 is true;

}
else
{

statement(s) executed if both expressions are false;

}

unless (expression

)
{

statement(s) executed if expression is false;

}