Documentation for syntax BEGIN assembled from the following pages:

Language documentation: Phasers §

From Phasers

(Phasers) Phasers BEGIN BEGIN §

Runs at compile time, as soon as the code in the phaser has compiled, only runs once.

The return value is available for use in later phases:

say "About to print 3 things";
for ^3 {
    say ^10 .pick ~ '-' ~ BEGIN { say  "Generating BEGIN value"; ^10 .pick }
}
# OUTPUT: 
# Generating BEGIN value 
# About to print 3 things 
# 3-3 
# 4-3 
# 6-3

The ^10 .pick in the phaser is generated only once and is then re-used by the loop during runtime. Note how the say in the BEGIN block is executed before the say that is above the loop.