require always returns a true value when it succeeds. (You know how Perl modules have to end in a true value? It returns that; though repeated use of require to load the same module just returns 1.) So the "1" in your code is completely unnecessary.

eval { require Data::Dump } or require Data::Dumper;
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anyway:

package MyModule;
use strict;
use warnings;

eval { require Data::Dump; 1} or require Data::Dumper;

_mydump('this is %INC',\%INC);

sub _mydump{
    my ( $msg, $ref) = @_;
    print "$msg:\n";
    {
        local $@;
        eval {dd $ref};
        print Dumper $ref if $@;
    }

}

#out

perl sopw.pl

Name "MyModule::Dumper" used only once: possible typo at sopw.pl line 
+15.
this is %INC:
print() on unopened filehandle Dumper at sopw.pl line 15.
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L*

Hi Discipulus,

The last message is precedence with print() -- the first is because you don't restrict that code path to the appropriate context. You could use goto conditionally:

package MyModule;
use strict;
use warnings;

my $Dump;
if (eval { require Data::Dump; 1}) {
    $Dump = sub { goto &Data::Dump::dd };
} else {
    require Data::Dumper;
    $Dump = sub { goto &Data::Dumper::Dumper };
}

print "this is %INC:\n";
print $Dump->(\%INC);

__END__
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Output:

this is %INC:
{
  "Data/Dump.pm"         => "/usr/share/perl5/Data/Dump.pm",
  "Exporter.pm"          => "/usr/share/perl/5.26/Exporter.pm",
  "overload.pm"          => "/usr/share/perl/5.26/overload.pm",
  "overloading.pm"       => "/usr/share/perl/5.26/overloading.pm",
  "strict.pm"            => "/usr/share/perl/5.26/strict.pm",
  "subs.pm"              => "/usr/share/perl/5.26/subs.pm",
  "vars.pm"              => "/usr/share/perl/5.26/vars.pm",
  "warnings.pm"          => "/usr/share/perl/5.26/warnings.pm",
  "warnings/register.pm" => "/usr/share/perl/5.26/warnings/register.pm
+",
}
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The way forward always starts with a minimal test.