It's a subscript, like used with an array, or in this case, a list.

my $value = ('a', 'b', 'c', 'd')[1];
say $value;
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...prints b

Very convenient as a quick and dirty tool in situations like this:

my $keep = ( split /\s+/, $string )[2];
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...although as with most things in Perl, that's only one of many ways to grab the third space-delimited item in a string.

This method must return a list, and the fifth element was the only thing useful to the caller. Expanding it out, it may make more sense.

my $bg = ($c->itemconfigure(qw/current -background/))[4];
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my @values = $c->itemconfigure(qw/current -background/);
my $bg = $values[4];
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You often see this for builtin functions like stat() or localtime(). They return lists of potentially useful stuff, but you may only need one thing. The documentation for each one clearly shows the meaning of each returned value in a list of returned values.

... you may only need one thing.

And if you need more than one thing, that can be handled, too:

>perl -wMstrict -le
"my @ra =
   (qw(start a b c e l p r y z end))[-1, 6, 4, 7, 5, 1..3, 0];
 ;;
 printf qq{'$_' } for @ra;
"
'end' 'p' 'e' 'r' 'l' 'a' 'b' 'c' 'start'
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It's (part) of a list slice. List slices are documented in perldata.

[ Of the three existing posts, neither of those facts were mentioned!?! ]

A list slice has the following form:

( EXPR )[ EXPR ]
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Of the scalars returned by the left EXPR, the slice returns those identified by the indexes returned by the right EXPR.

( 'foo', 'bar', 'baz' )[ 0, 2, 4 ]   # 'foo', 'baz', undef
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The scalars are returned in the order identified by the indexes. Negative indexes index from the end like with arrays. (-1 = last, -2 = second last, etc)

Thank you to all who replied. I am always surprised to see the various syntaxes/constructs in perl that are not used regularly or even mentioned in books...

The itemconfigure() method is documented in Tk::Canvas (see the WIDGET METHODS section).

You'll also find it in the book Mastering Perl/Tk (see Chapter 9: The Canvas Widget).