First of all, all lowercase module names are reserved for perl pragmas. You better name your modules "Interface1" and "Interface2" respectively.

For example, if I call the function terraform which is described in both interfaces and the variable $version is 1, then the interface1.pm version of terraform will be used. If version was 2, interface2.pm would be used.

If I get you right you want something like this:

#file Interface1.pm
package Interface1;

sub foo
{
    return "I am one!\n";
}

1;

#file Interface2.pm
package Interface2;

sub foo
{
    return "I am two!\n";
}

1;

#file script.pl
use strict;
use warnings;

use Interface1;
use Interface2;

my $interface;

$interface = "Interface1";
print $interface->foo();

$interface = "Interface2";
print $interface->foo();
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Edit:
or without "class method syntax":

$interface = "Interface1";
eval "print $interface" . "::foo()";

$interface = "Interface2";
eval "print $interface" . "::foo()";
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Note that you have to use the concatenation here, because perl would interpret

eval "print $interface::foo()";
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as a reference to the variable $foo in package Interface1 and that fails.