If($user!~/регул.выражение/) - как объясняется это условие?

our $buf;
 
{ 
  local $/ = undef;
  $buf = readline;
}
 
print $buf;

sub fold(&$@) {
 
    my ($fn, $x, @xs) = @_;
    my ($y, @ys) = @xs;
 
    local $a = @ys ? fold($fn, $x, @ys) : $x;
    local $b = $y;
 
    $fn->($a, $b);
};
 
# fold {$a + $b} 0, (1..3)  ==  fold {$_[0] + $_[1]} 0, (1..3)
# fold {$a * $b} 1, (1..3)  ==  fold {$_[0] + $_[1]] 1, (1..3)

sub fold(&$@) {
 
    my ($fn, $x, @xs) = @_;
    my ($y, @ys) = @xs;
 
    $a = @ys ? fold($fn, $x, @ys) : $x;
    $b = $y;
 
    $fn->($a, $b);
};
 
# fold {$a + $b} 0, (1..3)  ==  fold {$_[0] + $_[1]} 0, (1..3)
# fold {$a * $b} 1, (1..3)  ==  fold {$_[0] + $_[1]] 1, (1..3)

$a =1; $b = 2;
{ local ($a,$b) = 100,200; }  
 
print "$a $b";