The return value of a for statement is the empty string, as the following program shows:

#!/usr/bin/perl

use strict;
use warnings;
               
my $x = do {for (1 .. 4) {}};
my @x = do {for (1 .. 4) {}};

printf "Scalar return value of for is %s\n",
               defined $x ? "defined" : "undefined";
printf "Scalar return value of for is %s\n",
               $x ? "true" : "false";
printf "Scalar return value of for is has length %d\n",
               length $x;
printf "Scalar return value of for is '%s'\n",
               $x;

printf "List return value of for returns %d element\n",
               scalar @x;
printf "List return value of for contains %d undefined elements\n",
               scalar grep {!defined} @x;
printf "List return value of for contains %d true elements\n",
               scalar grep {$_} @x;
printf "List return value of for contains %d elements of non-zero leng
+th\n",
               scalar grep {length} @x;
printf "List return value of for equals [%s]\n",
               join ", ", map {"'$_'"} @x;

__END__
Scalar return value of for is defined
Scalar return value of for is false
Scalar return value of for is has length 0
Scalar return value of for is ''
List return value of for returns 1 element
List return value of for contains 0 undefined elements
List return value of for contains 0 true elements
List return value of for contains 0 elements of non-zero length
List return value of for equals ['']
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Note also that while Perl makes a difference between functions and statements (mostly for the benefit of mortals), perl doesn't. Operators, functions, statements, they're all opcodes. Here's the handling of the leaveloop opcode:

PP(pp_leaveloop)
{
    dSP;
    register PERL_CONTEXT *cx;
    I32 gimme;
    SV **newsp;
    PMOP *newpm;
    SV **mark;

    POPBLOCK(cx,newpm);
    mark = newsp;
    newsp = PL_stack_base + cx->blk_loop.resetsp;

    TAINT_NOT;
    if (gimme == G_VOID)
        ; /* do nothing */
    else if (gimme == G_SCALAR) {
        if (mark < SP)
            *++newsp = sv_mortalcopy(*SP);
        else
            *++newsp = &PL_sv_undef;
    }
    else {
        while (mark < SP) {
            *++newsp = sv_mortalcopy(*++mark);
            TAINT_NOT;          /* Each item is independent */
        }
    }
    SP = newsp;
    PUTBACK;

    POPLOOP(cx);        /* Stack values are safe: release loop vars ..
+. */
    PL_curpm = newpm;   /* ... and pop $1 et al */
 
    LEAVE;
    LEAVE;
    
    return NORMAL;
}
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You don't have to understand what it does, but do note that it's looking at its context to determine what to leave behind on the stack.

And here is a trace:

perl -Dt -e '$x = do {for (1) {}}'

EXECUTING...

(-e:0)  enter
(-e:0)  nextstate
(-e:1)  pushmark
(-e:1)  const(IV(1))
(-e:1)  gv(main::_)
(-e:1)  enteriter
(-e:1)  iter
(-e:1)  and
(-e:1)  stub
(-e:1)  unstack
(-e:1)  iter
(-e:1)  and
(-e:1)  leaveloop
(-e:1)  gvsv(main::x)
(-e:1)  sassign
(-e:1)  leave
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