#!/usr/bin/perl
use strict;
use warnings;
use diagnostics;

if (my $x = 1) {
#
} elsif (my $x = 2) {
#
}

##</code><code>##

"my" variable $x masks earlier declaration in same scope at test.pl line 9 (#1) (W misc) A "my" or "our" variable has been redeclared in the current scope or statement, effectively eliminating all access to the previous instance.  This is almost always a typographical error.  Note that the earlier variable will still exist until the end of the scope or until all closure referents to it are destroyed.

Found = in conditional, should be == at test.pl line 9 (#2)
    (W syntax) You said

        if ($foo = 123)

    when you meant

        if ($foo == 123)

    (or something like that).

##</code><code>##

#!/usr/bin/perl
use strict;
use warnings;
use diagnostics;

# set $x from get_num(), if it returns true value
# then do something with $x
if (my $x = get_num()) {
  print $x, "\n";
} else {
  print "empty\n";
}

sub get_num { 3 }

##</code><code>##

if ( (my $x = get_num()) >= 3 ) {
 ...
}

##</code><code>##

my $x = get_num(); # assumed to always returns a number
if ($x < 0 ) {
  # negative numbers
} elsif ($x < 3) {
  # less
} else {
  # OK
}