How about:

#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11160391
use warnings;

# wanted
#my $namematch = defined $options{r} ? qr($options{r}) : qr{s/\.[^.]+$
+//r};
#my @name = $file =~ $namematch;

my $file = 'foobar.txt';
my %options = ( r => '(.{3})', other => '.{3}(.{3})' );

# case for defined
{
my $namematch = defined $options{r} ? sub { /$options{r}/ } : sub { s/
+\.[^.]+$//r };
my @name = map { $namematch->() } $file;
print "defined case, \@name = @name\n";
}

# case for not defined
{
%options = ();
my $namematch = defined $options{r} ? sub { /$options{r}/ } : sub { s/
+\.[^.]+$//r };
my @name = map { $namematch->() } $file;
print "not defined case, \@name = @name\n";
}
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Outputs:

defined case, @name = foo
not defined case, @name = foobar
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Avoids the eval