The error message Variable "%s" will not stay shared is described in perldiag. The description is brief but explains the behavior you experienced. You can also read about closure in perlref, perlsub and elsewhere.

This is why you don't nest named subs, the closure becomes "static"

perl -E " sub f{ my %g; sub g{say \%g; } g(); say \%g; \%g } say f(),$/ for 1 .. 4;"

perl -E ' sub f{ my %g; sub g{say \%g; } g(); say \%g; \%g } say f(),$/ for 1 .. 4; '

HASH(0xabd9c4)
HASH(0xabd9c4)
HASH(0xabd9c4)

HASH(0xabd9c4)
HASH(0x3f9b6c)
HASH(0x3f9b6c)

HASH(0xabd9c4)
HASH(0x3f9b8c)
HASH(0x3f9b8c)

HASH(0xabd9c4)
HASH(0x3f9b6c)
HASH(0x3f9b6c)

By making the nested subroutine a variable (also known as an anonymous sub), the closure is dynamic

$ perl -lE " sub f{ my %g; my $g = sub {say \%g; $g{1}++}; $g->(); say \%g; \%g } say f(),$/ for 1 .. 4;"

HASH(0xabd9d4)
HASH(0xabd9d4)
HASH(0xabd9d4)

HASH(0x3f9b1c)
HASH(0x3f9b1c)
HASH(0x3f9b1c)

HASH(0x99b374)
HASH(0x99b374)
HASH(0x99b374)

HASH(0x99b344)
HASH(0x99b344)
HASH(0x99b344)