Rather than change the tiny operation used to generate a list of letters, just avoid generating a list of letters at all:

sub tye2 {
    my( $sentence, $wantedLetters )= @_;
    while( $wantedLetters =~ /(.)/gs ) {
        return 0 if -1 == index($sentence,$1);
    }
    return 1;
}
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If you want to go for ugly code for the sake of micro-optimizations, then

sub tye1 {
    my( $sentence, $wantedLetters )= @_;
    -1 == index($sentence,$1) && return 0
        while( $wantedLetters =~ /(.)/gs );
    return 1;
}
[download]

Or get even uglier to the point of risking improper behavior in some cases:

sub tye0 {
    -1 == index( $_[0], $1 )  &&  return 0
        while( $_[1] =~ /(.)/gs );
    return 1;
}
[download]