Some additional comment on this: you really don't want $picture to be set outside the loop. It would make debbuging really hard.

As [id://dave_the_m] said: it is an alias. Meaning if you do $picture = "something else"; then the original value will change. Limiting the effect of the alias to the foreach-block (and everything that is called via the foreach-block and still is in the lexical-scope of the variable) is a really good thing. (Because it reduces the code you need to look at when debugging.)

Also note that it is not last that undefines it. For example: for my $x (0 .. 3) { } print $x;==> it won't print 3.

(Ofcourse in your example you won't see the difference after doing an assignment because you are using a temporary list.)(It does matter. Sort seems to return an alias.)

Update: added comment about last
Update2: stroke some incorrect text.

Just add one thing, with use strict and warnings, Perl does give error: (I added my in front of $picture, so that the error at line 15 is isolated)

use strict;
use warnings;

my $match = '1995_Olga_goes_to_Egypt';
my @pictures = qw{1995/1995_Olga_first_time_in_Cyprus/06330004.JPG
1995/1995_Olga_goes_to_Egypt/QD0017004.JPG};
foreach my $picture (sort @pictures) {
    print "Debug: $picture\n";
    if ($picture =~ m/$match/)
    {
      print "Matched, $picture\n";
      last;
    }
}
print "Result: $picture\n";
[download]

This gives:

Global symbol "$picture" requires explicit package name at math1.pl li
+ne 15.
Execution of math1.pl aborted due to compilation errors.
[download]

That's not relevant to the issue here. The following strict-compliant program illustrates the issue:

use strict;
use warnings;
my $x;
for $x (1 .. 3) { last if $x == 2 }
print $x;
[download]

which prints

Use of uninitialized value in print at - line 5.
[download]

On the contrary, it's very relevant. In fact, the original question is I think a great example of the importance of using strict and warnings.

In the original code, the loop variable $picture was not pre-declared outside the loop using my (as you did for your $x). Had the OP run his code under use strict;, it would have died at compile time, perhaps prompting the OP to take a closer look at his code - exactly the thing I want use strict; to do for me.

Even with strict-compliant code similiar to yours, the OP would have gotten an informative message, rather then just an empty string, when using use warnings; - namely, the Use of uninitialized value in print message. There is it, staring him in the face - $picture is not initialized.

It's true that the warning does not explain why there is a problem. But in my experience, understanding exactly where the problem lies, is half-way to solving it.