strict is a compile time pragma, it's effect is only on how the code in it's scope gets compiled, it doesn't do anything at runtime, so it will only be run once, even inside a loop. Actually now that I think of it any use statement only gets run once no matter where it is.

Before I realised that it's a use and so has no runtime effect, I did a bit of digging around in strict. The following might interest you.

If you look at

> perl -MO=Terse -e 'use strict;my @a;for my $f (@a){no strict "refs";
+print $$f}'
LISTOP (0x8225988) leave [1] 
    OP (0x831eec0) enter 
    COP (0x8245398) nextstate 
    OP (0x82258c8) padav [1] 
    COP (0x831ee58) nextstate 
    BINOP (0x831ee30) leaveloop 
        LOOP (0x8232a98) enteriter [2] 
            OP (0x82dc988) null [3] 
            UNOP (0x8232a70) null [141] 
                OP (0x823bc98) pushmark 
                OP (0x823bce8) padav [1] 
        UNOP (0x831ee10) null 
            LOGOP (0x82dc9e8) and 
                OP (0x82dc9a8) iter 
                LISTOP (0x82dc960) lineseq 
                    COP (0x823c348) nextstate 
                    LISTOP (0x823c300) print 
                        OP (0x823c328) pushmark 
                        UNOP (0x8225880) rv2sv 
                            OP (0x82c1c20) padsv [2] 
                    OP (0x82dc9c8) unstack
[download]

you can't see any sign of the no strict 'refs'. It's not in the op tree and therefore doesn't do anything at runtime.

If you look in more detail and compare the output of

perl -MO=Debug -e 'use strict;my @a;for my $f (@a){no strict "refs";pr
+int $$f}'
[download]

with

perl -MO=Debug -e 'use strict;my @a;for my $f (@a){print $$f}'
[download]

you will find (amongst other things) that the first contains

UNOP (0x83085c8)
        op_next         0x830b190
        op_sibling      0x0
        op_ppaddr       PL_ppaddr[OP_RV2SV]
        op_targ         0
        op_type         15
        op_seq          7139
        op_flags        6
        op_private      1
        op_first        0x8309088
[download]

and the second contains

UNOP (0x982ca28)
        op_next         0x981f960
        op_sibling      0x0
        op_ppaddr       PL_ppaddr[OP_RV2SV]
        op_targ         0
        op_type         15
        op_seq          7128
        op_flags        6
        op_private      3
        op_first        0x9835dd8
[download]

(I am far from an expert on these things by the way) RV2SV means Reference value 2 Scalar Value, this comes from $$f above. If you look in perl.h you'll find

#define HINT_STRICT_REFS 0x00000002 /* strict pragma */
[download]

This is used to turn on and off a flag in op_private.You can see that in the two debug dumps above, op_private is 1 for the no strict version and 3 for the all strict. This flag tells the interpreter whether the dereference should be checked for strictness when it's run. So here you can see exactly what the compile time effect of strict is.

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