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$ perl -Mstrict -Mwarnings -E 'my $foo; say ++$foo'
1
$ perl -Mstrict -Mwarnings -E 'my $foo; say $foo + 1'
Use of uninitialized value $foo in addition (+) at -e line 1.
1

foreach my $employee (@employees) {
    if ( $employee->salary < $threshold ) {
        increase_salary( $employee, 3_000 );
    }
}

if ( defined $threshold ) {
    foreach my $employee (@employees) {
...
        }
    }
}

use Unknown::Variables;

...
my @array   = ( 1, 2, 3, $value, 4, 5 );
my @less    = grep { $_ < 4 } @array;   # assigns (1,2,3)
my @greater = grep { $_ > 3 } @array;   # assigns (4,5)

foreach my $employee (@employees) {
    if ( $employee->salary < $threshold ) {
        increase_salary( $employee, 3_000 );
    }
}