$" $LIST_SEPARATOR

The $" variable, also called the $LIST_SEPARATOR indicates what goes between array elements when they are intepolated in a string.

examples/list_separator.pl

use strict;
use warnings;

my @planets = qw(
   Mercury
   Venus
   Earth
   Mars
   Jupiter
   Saturn
   Uranus
   Neptune
);

print "@planets\n";  # Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune

$" = '-';
print "@planets\n"; # Mercury-Venus-Earth-Mars-Jupiter-Saturn-Uranus-Neptune

{
    local $" = '-^-';
    print "@planets\n"; # Mercury-^-Venus-^-Earth-^-Mars-^-Jupiter-^-Saturn-^-Uranus-^-Neptune
}

print "@planets\n"; # Mercury-Venus-Earth-Mars-Jupiter-Saturn-Uranus-Neptune

$" = '';
print "@planets\n"; # MercuryVenusEarthMarsJupiterSaturnUranusNeptune

By default it contains a single space, but you can replace it by any othere string. Including the empty string.

Unless you'd like to to impact the whole code-base it is recommended to wrap the assignment in a block (with curly braces) and use local to localize the change. (You cannot use my on this variable.)

Mnemonic: works in double-quoted context.

See also the official documentation.