m/$mystrings[998]/;  # Fails
m/$mystrings[999]/;  # Works
m/$mystrings[1000]/; # Works
[download]

> perl -MO=Deparse x
Global symbol "$mystrings" requires explicit package name at x line 10
+.
x had compilation errors.
use strict 'refs';
my(@mystrings) = 'string' x 200;
$mystrings[99];
"$mystrings[100]";
/$mystrings[999]/;
/${'mystrings'}[998]/;
[download]

Update: These values also work: 111, 222, 333, 444, 555, 666, 777, 888. And apparently above 1000 it's not simple either. The first working ones after 1000 are: 1110, 1111, 1113, 1114, 1115, 1116, 1117, 1118, 1119, 1131, 1141, 1151, 1161, 1171, 1181, 1191, 1311, 1333, 1411

Overall, there's 393 values from 0 to 10,000 for which it works. The rest fail.

I found a work-around, just use a different array to index the string array instead of using numbers:

my @mystrings = ("string") x 200;
my @i = 0..$#mystrings;

m/$mystrings[$i[100]]/;
[download]

Thanks for your help. Should I report this issue?

Simpler workaround (or maybe it's the right thing to do in the first place) is this:

m/${mystrings[100]}/;
[download]

Your array only has 1 element:

my @mystrings = "string" x 200;
print 'size=', scalar @mystrings, "\n";
# size=1
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I don't know why the 1st regex does not produce an error, where the 2nd one does. But, maybe if you fix the warnings, you will get what you are looking for.

This doesn't seem to be the issue, as the same error occurs after you fix it (my @strings = ("string") x 200;).

    my $check = m/$mystrings[N]/;
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... on incrementing values of N:

#! /usr/bin/perl -w

###############
## Libraries ##
###############
use strict;
use warnings;
use File::Basename;
use B::Deparse;
use IO::File;


#############
## Globals ##
#############
my $iam = basename $0;
$| = 1;

my $b_verbose = 0;

my $text = q{
    use strict;
    use warnings;
    my @mystrings = ( "string" ) x 200;

    my $check = m/$mystrings[<NUM>]/;
};


##################
## Main Program ##
##################
for (my $i = 0; $i <= 1000; $i++) {
    try($i);
}


#################
## Subroutines ##
#################
sub try {
    my ($n) = @_;
    my $new = $text;
    $new =~ s/<NUM>/$n/g;
    my $fn = 'test.pl';
    my $fh = new IO::File($fn, "w") or die "Can't write '$fn' ($!)\n";
    print $fh "$new\n";
    close $fh;
    chomp(my @results = `perl -MO=Deparse $fn 2>&1`);
    my ($check) = grep { /check/ } @results;
    my $b_ok = ($check =~ /my \$check = \$mystrings\[\d+\]/)? 1: 0;
    if ($b_verbose) {
        printf "Value %4d => %s\n", $n, $b_ok? "Okay": "*** FAILS ***"
+;
    } else {
        printf "%s", $b_ok? '.': '@';
    }
}
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It prints '.' or '@' for "Okay" and "Failed" values of N, respectively.

You can also assign $b_verbose to a nonzero value to see more verbose results :)

Results (consistent with other reports) for ActiveState 5.8.9-827 and Strawberries 5.10.1.0 and 5.12.0.1 under Windows 7:

>perl -wMstrict -le
"my @mystrings = ('string') x 10_000;
 ;;
 my %evaled;
 for my $i (0 .. $#mystrings) {
   eval qq{ qr{\$mystrings[$i]} };
   $evaled{$i} = $@;
   }
 ;;
 my $ok =()= grep !$_, values %evaled;
 printf qq{ok  == %4d \n}, $ok;
 my $bad =()=
   grep $_ &&
        /Global symbol \"\$mystrings\" requires explicit package name/
+,
   values %evaled
   ;
 printf qq{bad == %4d \n}, $bad;
"
ok  ==  392
bad == 9608
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