sub interpolate {
    my ($text, %vars) = @_;
    $text =~ s!\$(\w+)|\$\{(\w+)\}! 
                 my $name = $1 || $2;
                 exists $vars{ $name } ? $vars{ $name } : '$' . $name
              !ge;
    return $text
};

print interpolate(
    'Hello $user, this is $var1. We also use the unknown variable $foo.',
    var1 => 'The value of var1',
    user => 'Je55eah',
    # ...
);