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

##</code><code>##

    use strict;
    use warnings;

    my %Undef = ( Show => 'okay', Go => 'maybe', );

    foreach my $key ( keys %Undef )
    {    if ( ! defined $Undef{$key} ) { print "1. $key value is undefined!\n"; }
    }

    $Undef{Salary} = undef;

    foreach my $key ( keys %Undef )
    {    if ( ! defined $Undef{$key} ) { print "2. $key value is undefined!\n"; }
    }

    open ( my $file, ">","./ondisk") || die " ! open |./ondisk| $!\n";
    foreach my $key ( keys %Undef )
    {    print $file "$key\t$Undef{$key}\n";         ## line 20
    }
    close $file;
    my %Defined; print "\n";
    open ( $file, "<","./ondisk") || die " ! open |./ondisk| $!\n";
    while ( my $value = <$file> )
    {    chomp $value;
        my ( $newkey, $newvalue ) = split(/\t/,$value);
        $Defined{$newkey} = $newvalue;
    }
    close $file;

    foreach my $key ( keys %Defined )
    {    if ( defined $Defined{$key} ) { print "$key\t|$Defined{$key}|\n"; }
    }

1;

##</code><code>##

> perl  unknown.plx

2. Salary value is undefined!
Use of uninitialized value $Undef{"Salary"} in concatenation (.) or string at unknown.plx line 20.

Go      |maybe|
Salary  ||
Show    |okay|